Effect of Temperature on the Composition of a Synthetic Hydrocarbon Aviation Lubricating Oil

Synthetic hydrocarbon aviation lubricating oils (SHALOs) gradually degrade over time when subjected to high temperatures, resulting in their composition and properties varying over the operation lifetime. Therefore, understanding the SHALO degradation properties by elucidating the mechanism on a molecular level, as a function of high temperature, is of interest. A SHALO was subjected to thermal treatment (TT) at 180, 200, 230, 250, 270, or 300 °C for 2 h. The chemical compositions of six TT samples and one fresh oil were analyzed by fourier transform infrared F spectroscopy, advanced polymer chromatography, and gas chromatography/mass spectrometry. Furthermore, the physicochemical properties, such as kinematic viscosity, pour point, and acid number, of seven samples were determined. The oil samples were grouped by cluster analysis (CA) using a statistical method. The SHALO was identified to comprise 20 functional groups, including comb-like alkanes, long-chain diesters, amines, phenols, and other compounds. TT at <230 °C caused partial cracking of the SHALO base oils, with a concomitant change in the antioxidant content and type, and the polycondensation reactions were dominant. The observed antioxidant changes were not obvious from TT at >230 °C. A large number of small-molecule compounds were detected, including n-alkanes and olefins. TT at 250 °C was shown to be an important threshold for the kinematic viscosity, pour point, and acid number of the samples. Below 250 °C, the sample properties were relatively stable; but at elevated TT temperatures (>250 °C), the properties were observed to dramatically degrade. As the sample color was highly sensitive to temperature, the TT temperature induced rapid and significant color changes. The CA analysis results for the oil compounds at the molecular level were in good agreement with observed changes in the physicochemical properties at the macro level.


Introduction
Synthetic lubricating oils (SLOs) are liquid lubricants, possessing specific structures and properties derived from organic synthesis. Compared with vegetable oils and mineral-based lubricating oils, SLOs have the advantages of eco-compatibility, excellent thermal stability, and low-temperature behavior [1,2]. There are six types of SLOs, namely, synthetic hydrocarbon oils, organic acid ester synthetic oils, polyether synthetic oils, phosphate esters, fluorine-containing oils, and silicone oils. The SLO types exhibit significant differences in chemical structure, lubricating properties, and antidegradation behaviors compared with the base oil. Therefore, specific SLOs are better suited to different working

Subsection FT-IR Analysis
As shown in Figure 1, there were no significant differences between the S 180 -S 300 and S rt FT-IR spectra. Twenty functional groups were detected in the FT-IR spectra (Table 1). Some functional groups exhibited absorbances attributed to aliphatic moieties at~2854,~2925, and 2957 cm −1 , C=O vibrations at~1741 cm −1 , CH 2 vibrations at~1465 cm −1 , -CH(CH 3 ) 2 vibrations at~1378 cm −1 , =CH-H vibrations at~970 cm −1 , and -(CH 2 ) n -vibrations at~720 cm −1 , which were significantly stronger than others, implying that the components in the oil samples mainly comprised these groups.
The bands at 2957, 2925, and 2854 cm −1 were attributed to aliphatic -CH 3 and -CH 2 stretching vibrations, the band at~1465 cm −1 was attributed to -CH 2 shear bending vibrations, and the band at 1378 cm −1 was assigned to the C-H deformation vibration in -CH(CH 3 ) 2 . A sharp and strong band was observed at 721 cm −1 , implying that samples were rich in four or more -CH 2 moieties. Functional groups detected in the samples indicated an abundance of long-chain aliphatic species.
The peak at 1741 cm −1 was assigned to the C=O stretching vibration in saturated fatty acid esters (ν C=O ). The peaks at 1169 and 1142 cm −1 were attributed to the C-O-C stretching vibrations in long-chain fatty acid esters (ν as (COC) ), suggesting an abundance of long-chain fatty acid esters present in the sample. The weak peaks located at~2730 and~2672 cm −1 were assigned to the -CHO stretching and C-H bending vibrations of the aldehyde group, respectively. The bands located at~1611 and~1588 cm −1 were attributed to aromatic ring skeletal vibrations (ν), while those at~879,~821, and~779 cm −1 corresponded to o-, m-, and p-substituents on the aromatic ring, respectively. The peak at 1506 cm −1 was assigned to the N-H bending vibrations (ν NH ) in secondary amines. The C-N stretching vibrations (ν C-N ) of aromatic secondary amines produced bands at 1241 and 1314 cm −1 . The peaks observed at FT-IR spectra of Srt, S180, S200, S230, S250, S270, and S300. Table 1. FT-IR spectra assignments for Srt, S180, S200, S230, S250, S270, and S300. The bands at 2957, 2925, and 2854 cm −1 were attributed to aliphatic -CH3 and -CH2 stretching vibrations, the band at ~1465 cm −1 was attributed to -CH2 shear bending vibrations, and the band at 1378 cm −1 was assigned to the C-H deformation vibration in -CH(CH3)2. A sharp and strong band was observed at 721 cm −1 , implying that samples were rich in four or more -CH2 moieties. Functional groups detected in the samples indicated an abundance of long-chain aliphatic species.
The peak at 1741 cm −1 was assigned to the C=O stretching vibration in saturated fatty acid esters (C=O). The peaks at 1169 and 1142 cm −1 were attributed to the C-O-C stretching vibrations in longchain fatty acid esters ( as (COC)), suggesting an abundance of long-chain fatty acid esters present in the  As shown in Figure 1 and Table 1, the FTIR spectra of the seven oil samples changed very little under high-temperature treatment. Strong absorbances at around 2957, 2925, 2854, 1741, 1465, 970, and 721 cm −1 suggested that long-chain alkanes, carbonyl compounds, and unsaturated hydrocarbons were still abundant in the aged oils. The O-H stretching vibration located at~3645 cm −1 decreased as a function of TT temperature until the vibration was no longer detected, indicating that phenolic antioxidants were continuously consumed. The C=C stretching vibration observed at~1640 cm −1 was only detected in S 300 , which suggested that long-chain hydrocarbons were broken into olefins (OFs) and other substances as a function of TT temperature. Thermal cracking and polymerization reactions were concluded to occur to varying degrees during the high-temperature treatment of SHALOs, with antioxidant consumption somewhat preventing the lubricating oil properties from diminishing.

APC Analysis
The relative molecular mass and corresponding mass distribution are among the most basic structural parameters of lubricating oils that are closely related to the physicochemical properties (such as KV and PP) and the thermal and tribological properties of the lubricating oil. The molecular mechanism of thermal stability can be explored using the molecular weight and corresponding mass distribution. APC is an effective method for measuring the molecular weight and molecular weight distribution of lubricating oils. APC can also obtain important molecular weight information, such as the weight-average molecular weight (M w ) and number-average molecular weight (M n ).
The SHALO used in this study contained a base oil comprising 80% PAO, 20% diisooctyl adipate (DIOA), and a small number of additives. A previous study [36] has shown that PAO is polymerized and comprises linear α-olefins ranging from trimers to pentamers. The PAO structure is relatively regular with a uniform comb-shaped isoparaffin (IP) chain length of generally C 8 -C 10 . Based on the M w of S rt being 507 Da (Figure 2), the M w of PAO could be predicted to be 520 Da, with a molecular formula of C 37 H 76 . Combined with the PAO comb-shaped structure, the preliminary estimated structural formula of PAO is shown in Figure 3.

GC/MS Analysis
As shown in Figure 4, Srt was rich in two base oils (PAO and DIOA) and also contained four additives, three of which were antioxidants (butylated hydroxytoluene (BHT), N-phenyl-naphthylamine (NPAN), 4-octyl-N-(4-octylphenyl) aniline (ODA)), while the other was an antiwear The highest M n and M w values of 501 and 511 Da, respectively, were observed at 230 • C. Thereafter, M n and M w were again reduced and found to be stable at temperatures over 230 • C, with values of 495 and 487 Da, 494 and 486 Da, and 494 and 486 Da for S 250 , S 270 , and S 300 , respectively. There was insufficient thermal energy in the SHALOs to induce a larger degree of polymerization below 200 • C, at which the M w was relatively low owing to the cracking degree being higher than the polymerization degree. In the range of 200-230 • C, increasing temperature caused larger-scale polymerization owing to TT, providing sufficient energy to significantly increase the M w . Therefore, M n and M w reached peak values in S 230 . The cracking degree was again higher than the polymerization degree when the temperature increased above 230 • C. In contrast, the SHALOs were less able to dissolve the polymers, resulting in coking and sludge formation at high temperatures. Under these experimental conditions, the M w first decreased, then increased, and finally decreased again. The molecular weight distribution width index, D (D = M w /M n ), varied from 1.018 to 1.020 depending on the TT temperature, showing that the oil samples were narrow heterogeneous dispersion systems at each temperature.  Figure 2. Molecular weight distributions of Srt, S180, S200, S230, S250, S270, and S300.

GC/MS Analysis
As shown in Figure 4, Srt was rich in two base oils (PAO and DIOA) and also contained four additives, three of which were antioxidants (butylated hydroxytoluene (BHT), N-phenyl-naphthylamine (NPAN), 4-octyl-N-(4-octylphenyl) aniline (ODA)), while the other was an antiwear additive (tris(3-methylphenyl) phosphate (TMP)). Figure 4 also shows significant differences in oil composition as a function of TT temperature. In contrast to Srt, DIOA and NPAN were not detected in S180-S300 at retention times of 7-11 min and 15-17 min by GC/MS, respectively. Significant compositional differences existed between Srt and S180, including a clear reduction in BHT, ODA, and TMP contents observed in S180. Furthermore, BHT and TMP were barely detected, together with an observed decrease in ODA in S300, implying that antioxidants were continuously removed from the system, preventing thermal cracking reactions. Furthermore, the compounds detected by GC/MS increased in intensity as a function of TT temperature over the first 11 min. The free radicals generated by thermal oxidation during the service life of lubricants are suggested to first be consumed by free-radical trapping antioxidant NPAN (medium-temperature antioxidant), which hinders more severe oxidation reactions [37] and, therefore, maintains the normal lubrication function of the oil.

GC/MS Analysis
As shown in Figure 4, S rt was rich in two base oils (PAO and DIOA) and also contained four additives, three of which were antioxidants (butylated hydroxytoluene (BHT), N-phenyl-α-naphthylamine (NPAN), 4-octyl-N-(4-octylphenyl) aniline (ODA)), while the other was an antiwear additive (tris(3-methylphenyl) phosphate (TMP)).  Cracking reactions were performed with the oil samples according to the variation and relative contents (RCs) of small-molecule compounds. As shown in Figure 5 and Table 2, 82 organic compounds were detected by GC/MS, which were classified into n-alkanes (NAs), eight IPs, 21 OFs, 24 ETs, three alcohols (AHs), eight antioxygen and other compounds (AOs), and two unknowns  Figure 4 also shows significant differences in oil composition as a function of TT temperature. In contrast to S rt , DIOA and NPAN were not detected in S 180 -S 300 at retention times of 7-11 min and 15-17 min by GC/MS, respectively. Significant compositional differences existed between S rt and S 180 , including a clear reduction in BHT, ODA, and TMP contents observed in S 180 . Furthermore, BHT and TMP were barely detected, together with an observed decrease in ODA in S 300 , implying that antioxidants were continuously removed from the system, preventing thermal cracking reactions. Furthermore, the compounds detected by GC/MS increased in intensity as a function of TT temperature over the first 11 min. The free radicals generated by thermal oxidation during the service life of lubricants are suggested to first be consumed by free-radical trapping antioxidant NPAN (medium-temperature antioxidant), which hinders more severe oxidation reactions [37] and, therefore, maintains the normal lubrication function of the oil.
Cracking reactions were performed with the oil samples according to the variation and relative contents (RCs) of small-molecule compounds. As shown in Figure 5 and Table 2, 82 organic compounds were detected by GC/MS, which were classified into n-alkanes (NAs), eight IPs, 21 OFs, 24 ETs, three alcohols (AHs), eight antioxygen and other compounds (AOs), and two unknowns (UKs).
The above results showed that temperature played an important role in the compounds detected and influenced the compound type and RC. As shown in Figures 4 and 5, at a retention time of 8-11 min, the RCs of DIOA and NPAN in S rt were high, but the same compounds were not detected in S 180 . Instead, BHT underwent basic structure unit degradation to yield a small number of hydrocarbons. (Z)-Oct-2-ene (4, C 8 ), NAs (6, C 10 ; 21, C 15 ), and 4-methylheptan-1-ol (7, C 8 ) were observed in S 200 , while the other two BHT derivatives, namely, 2-(tert-butyl)-4-methylphenol (12) and the isomer of BHT (24), were detected in S 230 . When the temperature reached 250 • C, small-molecule compounds showed increased contents, with the appearance of additional NAs (5, C 9 ; 6, C 10 ; 8, C 11 ; 10, C 12 ; 21, C 15 ; 26, C 16 ). As shown in Figures 4 and 6, when the temperature exceeded 270 • C, the amount and RC of small molecular compounds in S 300 gradually increased compared with the other TT oil samples, reaching 52 compounds and 4.600%, respectively. This might be principally attributed to thermal cracking reactions of the base oils. Conversely, BHT, ODA, and TMP cracked only into relatively few small molecules through high-temperature oxidation. The results showed that the oil samples subjected to TT temperatures of >230 • C presented an increase in the number of small molecules, while the average molecular weight decreased, with concomitant deterioration in the properties. These observations were in agreement with the APC results.                     molecular compounds in S300 gradually increased compared with the other TT oil samples, reaching 52 compounds and 4.600%, respectively. This might be principally attributed to thermal cracking reactions of the base oils. Conversely, BHT, ODA, and TMP cracked only into relatively few small molecules through high-temperature oxidation. The results showed that the oil samples subjected to TT temperatures of >230 °C presented an increase in the number of small molecules, while the average molecular weight decreased, with concomitant deterioration in the properties. These observations were in agreement with the APC results.  Figure 6. Distribution of group components in Srt and S180-S300.

Physicochemical Properties: Changes with Temperature
As shown in Figure 7, the KV at 40 • C (ν 40 ), PP, AN, and color showed different variational trends as a function of temperature. Changes in the trends of ν 40 , PP, and AN occurred at an inflection point of 250 • C. When the temperature was <250 • C, the ν 40 , PP, and AN values of the samples were relatively stable. The ranges of variation for ν 40 , PP, and AN were 14.6-14.4 mm 2 /s, −65 to −69 • C, and 0.07-0.45 mg KOH/g, respectively. When the temperature was >250 • C, the ν 40 , PP, and AN values of the samples changed dramatically. The ν 40 values of S 270 and S 300 were reduced to 14.1 and 13.5 mm 2 /s, while the PP value was increased to −56 and −27 • C, and the AN value was increased to 0.8 and 3.5 mg KOH/g, respectively.
Materials 2020, 13, x FOR PEER REVIEW 7 of 22 As shown in Figure 7, the KV at 40 °C (40), PP, AN, and color showed different variational trends as a function of temperature. Changes in the trends of 40, PP, and AN occurred at an inflection point of 250 °C. When the temperature was <250 °C, the 40, PP, and AN values of the samples were relatively stable. The ranges of variation for 40, PP, and AN were 14.6-14.4 mm 2 /s, −65 to 69 °C, and 0.07-0.45 mg KOH/g, respectively. When the temperature was >250 °C, the 40, PP, and AN values of the samples changed dramatically. The 40 values of S270 and S300 were reduced to 14.1 and 13.5 mm 2 /s, while the PP value was increased to 56 and 27 °C, and the AN value was increased to 0.8 and 3.5 mg KOH/g, respectively.
As shown in Figure 7, the inflection point for color was 200 °C. When subjected to TT temperatures of >200 °C, the oil samples were brown and black, while those at 180 °C were light brown. Compared with 40, PP, and AN, the color was more sensitive to temperature, which induced rapid changes in the oil color. The color change preceded the degradation of the oil sample during the heating process and was relatively small during subsequent heating. Therefore, the color change should not be used as a basis for the degradation performance of the oil sample. Basing changes in the lubricating function of the oil solely on color changes is not ideal because the performance of the oil might not have actually deteriorated, resulting in waste. Studies have shown that changes in the oil color are related to the antioxidant effect of antioxidants [38]. The change in AN was derived from oxygen-containing compounds, such as alcohols, ketones, aldehydes, and acid esters, generated during high-temperature thermal oxidation [39]. PP is principally related to the -methylene content in alkyl groups and aromatic or naphthenic rings. The NAs exhibit a higher PP, while the IPs exhibit a lower PP [40]. During the low-and medium-temperature stages (<250 °C), the antioxidant effect, while changing the color of the oil samples, also prevented the cracking of the PAO and DIOA molecular chains in the oil samples and, to a certain extent, the formation of other substances [41]. Therefore, the degradation rate of 40, PP, and AN at this stage was reduced. At the high-temperature stage (>250 °C), the molecular structure of PAO was destroyed, and the type and content of n-alkanes exceeded that of o-alkanes, leading to an increase in PP [42]. Simultaneously, with the decrease in antioxidant content, the antioxidant effect was obviously weakened. The oil samples were typically subjected to high-temperature thermal oxidation and cracking, and the contents of small molecular oxygen-containing compounds, such as alcohols, ketones, aldehydes, and acid esters, significantly increased, leading to an increase in AN. The KV of the lubricating oil is mainly derived from the interaction force between molecules. The distance between the molecules increases with rising temperature owing to volume expansion, which results in the decrease in force between molecules and, therefore, a decrease in viscosity. The As shown in Figure 7, the inflection point for color was 200 • C. When subjected to TT temperatures of >200 • C, the oil samples were brown and black, while those at 180 • C were light brown. Compared with ν 40 , PP, and AN, the color was more sensitive to temperature, which induced rapid changes in the oil color. The color change preceded the degradation of the oil sample during the heating process and was relatively small during subsequent heating. Therefore, the color change should not be used as a basis for the degradation performance of the oil sample. Basing changes in the lubricating function of the oil solely on color changes is not ideal because the performance of the oil might not have actually deteriorated, resulting in waste. Studies have shown that changes in the oil color are related to the antioxidant effect of antioxidants [38]. The change in AN was derived from oxygen-containing compounds, such as alcohols, ketones, aldehydes, and acid esters, generated during high-temperature thermal oxidation [39]. PP is principally related to the α-methylene content in alkyl groups and aromatic or naphthenic rings. The NAs exhibit a higher PP, while the IPs exhibit a lower PP [40]. During the low-and medium-temperature stages (<250 • C), the antioxidant effect, while changing the color of the oil samples, also prevented the cracking of the PAO and DIOA molecular chains in the oil samples and, to a certain extent, the formation of other substances [41]. Therefore, the degradation rate of ν 40 , PP, and AN at this stage was reduced. At the high-temperature stage (>250 • C), the molecular structure of PAO was destroyed, and the type and content of n-alkanes exceeded that of o-alkanes, leading to an increase in PP [42]. Simultaneously, with the decrease in antioxidant content, the antioxidant effect was obviously weakened. The oil samples were typically subjected to high-temperature thermal oxidation and cracking, and the contents of small molecular oxygen-containing compounds, such as alcohols, ketones, aldehydes, and acid esters, significantly increased, leading to an increase in AN.
The KV of the lubricating oil is mainly derived from the interaction force between molecules. The distance between the molecules increases with rising temperature owing to volume expansion, which results in the decrease in force between molecules and, therefore, a decrease in viscosity. The small molecular compounds produced by cracking at high temperatures have made a greater contribution to the reduction in KV [43]. Based on the APC results, at >230 • C, the temperature was known to rapidly decrease both M n and M w . Therefore, the relative number of small-molecule compounds increased, which led to a decrease in ν 40 at high temperatures.

Molecular Structure: Changes with Temperature
The molecular structure of SLOs determines the thermal oxidation stability when oils are exposed to harsh operating conditions [2,44]. As shown in Figures 6 and 8, there were significant differences in the species and RCs of compounds detected by GC/MS between the fresh oil and aged oils. The fresh oil (S rt ) comprised 23 ETs (10.90%) and three AOs (0.913%), while hydrocarbons, alcohols, and AO compounds were the main compounds observed in the aged oils (S 180 -S 300 ). ETs were not detected in S 180 -S 270 , but pentylpentanoate was observed in S 300 . Degradation products were produced during the test, implying that the SHALO thermal oxidation process was highly complex. Meanwhile, any small changes in the experimental conditions, such as the experimental reactor, heating and cooling conditions, sampling approaches, and production batches of the same lubricating oils, could affect the results and the changing trend under high temperatures. small molecular compounds produced by cracking at high temperatures have made a greater contribution to the reduction in KV [43]. Based on the APC results, at >230 °C, the temperature was known to rapidly decrease both Mn and Mw. Therefore, the relative number of small-molecule compounds increased, which led to a decrease in 40 at high temperatures.

Molecular Structure: Changes with Temperature
The molecular structure of SLOs determines the thermal oxidation stability when oils are exposed to harsh operating conditions [2,44]. As shown in Figures 6 and 8, there were significant differences in the species and RCs of compounds detected by GC/MS between the fresh oil and aged oils. The fresh oil (Srt) comprised 23 ETs (10.90%) and three AOs (0.913%), while hydrocarbons, alcohols, and AO compounds were the main compounds observed in the aged oils (S180-S300). ETs were not detected in S180-S270, but pentylpentanoate was observed in S300. Degradation products were produced during the test, implying that the SHALO thermal oxidation process was highly complex. Meanwhile, any small changes in the experimental conditions, such as the experimental reactor, heating and cooling conditions, sampling approaches, and production batches of the same lubricating oils, could affect the results and the changing trend under high temperatures. The oxidation process of lubricating oils has three stages [26]. First, the antioxidant and antiwear additives are consumed, followed by oxidative degradation of the base oil, and, finally, polymerization, which leads to an increase in the oil viscosity in the final stage. The APC and GC/MS data were in agreement with the three stages of oxidation. The additives were depleted at 180 °C in The oxidation process of lubricating oils has three stages [26]. First, the antioxidant and antiwear additives are consumed, followed by oxidative degradation of the base oil, and, finally, polymerization, which leads to an increase in the oil viscosity in the final stage. The APC and GC/MS data were in agreement with the three stages of oxidation. The additives were depleted at 180 • C in the first stage. The RCs of the three antioxidants (BHT, NPAN, and ODA) and one antiwear additive (TMP) were reduced, but the amount of these compounds increased. Four derivatives of BHT are shown in Figure 9, and the number of carbon atoms associated with the NAs in S 180 was not less than 20. Generally, small-molecule compounds, which increased in content during degradation of the lubricating oil, resulted in decreased M n and M w values, while cross-linking and polycondensation of the compounds resulted in increased M n and M w values. Furthermore, decane was detected in S 180 . The oil samples at <200 • C were concluded to be partially pyrolyzed, as characterized by a growing amount of small-molecule compounds compared with that of macromolecules. The KV values of S 180 and S 200 were relatively low, suggesting that the formation of hydroperoxides in the initial oxidation period seriously deteriorated the properties of the aging oils. In the temperature range of 200-230 • C, the degree of polycondensation was greater than that of cracking in the aged oils owing to a growing volume of macromolecular compounds, while the KV of S 230 was observed to increase [45][46][47]. During this stage, the formation of polar organic compounds in the oil (such as ketones, alcohols, carboxylic acids, and esters) was more pronounced, with 4-methylheptan-1-ol and 2-(hexadecyloxy)ethan-1-ol detected. Furthermore, two new derivatives of BHT-2-(tert-butyl)-4-methylphenol and the isomer of BHT-were present in S 230 . The possible reaction phenolic hydroxyl oxygen pathways of BHT are shown in Figure 10. When heated, BHT acted as a radical terminator. The phenolic hydroxyl oxygen could participate in the benzene conjugated system to induce electronegativity, with the pushing of electron density from the o-, m-, and p-substituents resulting in a reduction of O-H polarity. Hydrogen atoms were quickly abstracted by the free radicals of the lubricating oils to improve the thermal oxidation stability. The change in the composition of the SHALO was similar to the three-stage oxidation process (depletion of additives, degradation of the base oil, and polymerization) when the TT temperature was <230 • C. In the temperature range of 250-300 • C, the degree of thermal cracking in the oil sample was greater than that of thermal polycondensation, characterized by a growing amount of small-molecule compounds. C 10 -C 18 compounds were detected in S 250 -S 300 , and the RC of BHT derivative 2-(tert-butyl)-4-methylphenol increased from 0.009% in S 230 to 0.646% in S 300 . Therefore, the KV of the aged oils was also significantly reduced [2,13]. Hydrogen atoms were quickly abstracted by the free radicals of the lubricating oils to improve the thermal oxidation stability. The change in the composition of the SHALO was similar to the threestage oxidation process (depletion of additives, degradation of the base oil, and polymerization) when the TT temperature was <230 °C. In the temperature range of 250-300 °C , the degree of thermal cracking in the oil sample was greater than that of thermal polycondensation, characterized by a growing amount of small-molecule compounds. C10-C18 compounds were detected in S250-S300, and the RC of BHT derivative 2-(tert-butyl)-4-methylphenol increased from 0.009% in S230 to 0.646% in S300. Therefore, the KV of the aged oils was also significantly reduced [2,13].   Overall, the polycondensation and cracking reactions of the oil samples were shown to occur simultaneously at high temperatures, with thermal cracking as the predominant mechanism of these reactions ( Figure 11). There was a significant difference between the degree of polycondensation and cracking reactions as a function of temperature. After polycondensation and pyrolysis reactions, the physicochemical properties of the oil samples changed, with the thermal stability decreasing to the degree that resulted in the deterioration of these properties. The results showed that at temperatures of <250 °C , the compounds in the base oil were slightly cracked, and the antioxidants significantly changed. Polycondensation is the predominant reaction below 250 °C, which might explain why lower wear was observed for the aged oils compared with fresh oil, and why the films formed from the aged oils provided superior wear performance compared with that of the film formed from fresh oil [24]. When the temperature exceeded 250 °C , the compounds present in the oil samples were mainly pyrolyzed, but the antioxidant effect was less obvious. Furthermore, the types of smallmolecule compounds increased, and a large number of small-molecule compounds, such as OFs and NAs, appeared. There were a large number of small molecular compounds, which indicated that a significant degree of thermal oxidation cracking occurred in the lubricating oil. Therefore, the molecular weight exhibited a downward trend, the viscosity decreased, and the oil film was not able to reach a certain thickness (≤0.01 m) [16]. The lubricating function of the lubricating oil was Overall, the polycondensation and cracking reactions of the oil samples were shown to occur simultaneously at high temperatures, with thermal cracking as the predominant mechanism of these reactions ( Figure 11). There was a significant difference between the degree of polycondensation and cracking reactions as a function of temperature. After polycondensation and pyrolysis reactions, the physicochemical properties of the oil samples changed, with the thermal stability decreasing to the degree that resulted in the deterioration of these properties. The results showed that at temperatures of <250 • C, the compounds in the base oil were slightly cracked, and the antioxidants significantly changed. Polycondensation is the predominant reaction below 250 • C, which might explain why lower wear was observed for the aged oils compared with fresh oil, and why the films formed from the aged oils provided superior wear performance compared with that of the film formed from fresh oil [24]. When the temperature exceeded 250 • C, the compounds present in the oil samples were mainly pyrolyzed, but the antioxidant effect was less obvious. Furthermore, the types of small-molecule compounds increased, and a large number of small-molecule compounds, such as OFs and NAs, appeared. There were a large number of small molecular compounds, which indicated that a significant degree of thermal oxidation cracking occurred in the lubricating oil. Therefore, the molecular weight exhibited a downward trend, the viscosity decreased, and the oil film was not able to reach a certain thickness (≤0.01 µm) [16]. The lubricating function of the lubricating oil was significantly impacted, with friction and wear occurring, leading to the deterioration of the oil properties.

CA of the Oil Samples
Clustering aims to seek natural groupings in the data based on the similarity of individual components or objects, such that the objects of the same group, or cluster, are more similar than different groups [48][49][50]. CA is considered a principal task of data exploration, is a common technique for statistical analysis and chemometric approaches, and is widely used in molecular spectroscopy [51][52][53]. The results of CA were intended to elucidate the compound structure and compositional changes of SHALOs resulting from oxidative high-temperature degradation, allowing detection of the degradation stages of SHALOs and signaling of the need for an oil change when warranted by the oil conditions [54]. Furthermore, the SHALO clusters could be used to detect possible relationships between compound structure and compositional changes at the molecular level and, therefore, facilitate the discovery of physicochemical properties at the macro level.
Based on 82 compounds detected from seven samples by GC/MS at retention times of 1-11 min, the SHALO clusters were classified by CA using Origin 9.0 software. The average distance between

CA of the Oil Samples
Clustering aims to seek natural groupings in the data based on the similarity of individual components or objects, such that the objects of the same group, or cluster, are more similar than different groups [48][49][50]. CA is considered a principal task of data exploration, is a common technique for statistical analysis and chemometric approaches, and is widely used in molecular spectroscopy [51][52][53]. The results of CA were intended to elucidate the compound structure and compositional changes of SHALOs resulting from oxidative high-temperature degradation, allowing detection of the degradation stages of SHALOs and signaling of the need for an oil change when warranted by the oil conditions [54]. Furthermore, the SHALO clusters could be used to detect possible relationships between compound structure and compositional changes at the molecular level and, therefore, facilitate the discovery of physicochemical properties at the macro level.
Based on 82 compounds detected from seven samples by GC/MS at retention times of 1-11 min, the SHALO clusters were classified by CA using Origin 9.0 software. The average distance between groups, data standardization, and the Pearson correlation coefficient was chosen for hierarchical CA. As shown in Figure 12, the system CA tree diagram showed that seven types of oils could be classified, as follows: S 180 , S 200 , S 230 , and S 250 belonging to cluster I, S 270 and S 300 belonging to cluster II, and S rt belonging to cluster III. The classifications of clusters I and II were consistent with the inflection points of the M n , M w , ν 40 , PP, and AN values of the oil samples at 250 • C. The classification of S rt into cluster III was also consistent with the physical system studied. CA results at the molecular level were observed to be in good agreement with the changes observed to the physicochemical properties of the samples on the macro level. Gong et al. observed that hierarchical CA results based on two-channel and differential dielectric spectroscopy (TD-DES) data were also in good agreement with those based on FT-IR data and proposed using the TD-DES technique to monitor the oil and conduct hierarchical CA [54]. Furthermore, the present study offered a platform for further researching the molecular mechanism of oil sample degradation performance.
for statistical analysis and chemometric approaches, and is widely used in molecular spectroscopy [51][52][53]. The results of CA were intended to elucidate the compound structure and compositional changes of SHALOs resulting from oxidative high-temperature degradation, allowing detection of the degradation stages of SHALOs and signaling of the need for an oil change when warranted by the oil conditions [54]. Furthermore, the SHALO clusters could be used to detect possible relationships between compound structure and compositional changes at the molecular level and, therefore, facilitate the discovery of physicochemical properties at the macro level.
Based on 82 compounds detected from seven samples by GC/MS at retention times of 1-11 min, the SHALO clusters were classified by CA using Origin 9.0 software. The average distance between groups, data standardization, and the Pearson correlation coefficient was chosen for hierarchical CA. As shown in Figure 12, the system CA tree diagram showed that seven types of oils could be classified, as follows: S180, S200, S230, and S250 belonging to cluster I, S270 and S300 belonging to cluster II, and Srt belonging to cluster III. The classifications of clusters I and II were consistent with the inflection points of the Mn, Mw, 40, PP, and AN values of the oil samples at 250 °C. The classification of Srt into cluster III was also consistent with the physical system studied. CA results at the molecular level were observed to be in good agreement with the changes observed to the physicochemical properties of the samples on the macro level. Gong et al. observed that hierarchical CA results based on two-channel and differential dielectric spectroscopy (TD-DES) data were also in good agreement with those based on FT-IR data and proposed using the TD-DES technique to monitor the oil and conduct hierarchical CA [54]. Furthermore, the present study offered a platform for further researching the molecular mechanism of oil sample degradation performance.

Conclusions
In this study, the compositional and structural characteristics of a SHALO were clarified under treatment at different temperatures through compound identification and analysis. The data provided scientific validation and technical support for molecular mechanisms related to the physicochemical properties and degree of SHALO deterioration as a function of temperature. Twenty functional groups, including comb-like alkanes, long-chain diesters, amines, and phenols, were identified in the SHALO. The antioxidants induced obvious effects that hindered the formation of free radicals, and NPAN was exhausted when the temperature was higher than 180 • C. Therefore, the cracking reactions were slower compared with polycondensation at low and medium temperatures (<250 • C). However, at high temperatures (>250 • C), the role of the antioxidants gradually weakened, which significantly promoted the cracking reactions. The number and relative contents of small-molecule compounds in S 300 , mainly composed of NAs, IPs, OFs, and AHs, were 52 and 4.600%, which were significantly higher compared with those in S 180 (15 and 2.228%). A correlation existed between the rapid degradation of physicochemical properties (such as color, KV, PP, and AN) and the rapid increase in small-molecule compounds under high-temperature treatment. CA, based on the RCs of compounds, was in good agreement with the classification based on macroscopic physicochemical properties, which revealed the molecular mechanism of the degradation in properties at high TT temperatures. Furthermore, on this basis, the high-temperature SHALO performance would be improved, and the service life prolonged by developing chemical analysis.