Thermal stability of aprotic ionic liquids as potential lubricants. Comparison with synthetic oil bases

Accepted Manuscript Thermal stability of aprotic ionic liquids as potential lubricants. Comparison with synthetic oil bases Juan J. Parajó, María Villanueva, Inés Otero, Josefa Fernández, Josefa Salgado PII: DOI: Reference:

S0021-9614(17)30328-2 http://dx.doi.org/10.1016/j.jct.2017.09.010 YJCHT 5209

To appear in:

J. Chem. Thermodynamics

Received Date: Revised Date: Accepted Date:

31 July 2017 5 September 2017 6 September 2017

Please cite this article as: J.J. Parajó, M. Villanueva, I. Otero, J. Fernández, J. Salgado, Thermal stability of aprotic ionic liquids as potential lubricants. Comparison with synthetic oil bases, J. Chem. Thermodynamics (2017), doi: http://dx.doi.org/10.1016/j.jct.2017.09.010

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Thermal stability of aprotic ionic liquids as potential lubricants. Comparison with synthetic oil bases Juan J. Parajó, María Villanueva, Inés Otero, Josefa Fernández, Josefa Salgado* Grupo Nafomat, Departamento de Física Aplicada, Facultade de Física, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain *[email protected]

1

Abstract Thermal stability of three ionic liquids (ILs) and five base stock lubricants was analysed using a Perkin Elmer thermogravimetric device and compared with that of the 19 ILs previously

studied.

The

ILs

are

1-(2-methoxyethyl)-1-methyl-pyrrolidinium

tris(pentafluoroethyl)trifluorophosphate,

[1-(2-methoxyethyl)-1-methyl-pyrrolidinium

bis(trifluoromethylsulfonyl)imide,

and

tris(pentafluoroethyl)trifluorophosphate,

and

hexaheptanoate,

a

polypropyleneglycol

trihexyl(tetradecyl)phosphonium the

bases

are

dimethylether,

dipentaerythritol and

three

perfluoroalkylpolyethers. Firstly, dynamic scans were performed, under air atmosphere and at a heating rate of 10 ºC · min-1, to estimate the short-term thermal stability. Anion influence on the shortterm thermal stability is higher than the cation one, and those ILs with bis(trifluoromethylsulfonyl)imide anion [NTf2 ]- are the most stable. All the ILs here studied showed higher on set temperatures than the five base oils. The influence of the water content on the short-term thermal stability was analysed through the on settemperature variation between two different water contents, as supplied and after saturation, observing that the influence of this impurity is not significant for the studied ILs. From isothermal experiments performed at temperatures lower than the on setones, the kinetic of the degradation was studied and the activation energy for the ILs and two lubricant bases was determined. The values obtained were used to estimate the maximum operation temperature of these fluids using three different methodologies. Finally, qualitative studies were performed based on the colour change after heating at different temperatures and on the character endo or exothermic of the DSC peaks associated to the different mass loss steps under N2 atmosphere. These studies showed that the degradation mechanism depends on the atmosphere.

Keywords: thermogravimetric analysis; ionic liquid; lubricant base; maximum operation temperature; degradation mechanism, differential scanning calorimetry.

2

1 Introduction Advances in lubricants have always played important roles to improve energy efficiency and durability. Lubricant industry is always trying to improve the performance of their products with the aim of increasing the friction coefficient reduction, the durability and service and to reduce emissions, among other objectives [1, 2]. The primary advantages of ILs over conventional lubricants lie in their better ability to form tribofilms, higher thermal and hydrolytic stability, lower vapour pressures, wider liquid range, higher thermal conductivity, lower combustibility environmental friendliness, and adaptability to various applications [3]. These are the main reasons why their use as high- and lowtemperature lubricants is among their potential technological applications of ILs [3, 4]. The thermal stability of lubricants is a parameter that limits the maximum service temperature, which is essential to be known for their applications at high temperatures. ILs and lubricants thermal stability can be affected by many parameters; in addition to the structure (cation and anion type, structural modifications in length and functionalities in cation alkyl chain) and impurities (water and chlorides) [5], the atmosphere and the exposition time play an important role in this property [6]. In general, perfluoropolyethers (PFPEs) have excellent thermal and oxidation stability with very low volatility, whereas polyolesters have also very few evaporation losses but the oxidation stability is lower than that of PFPEs. Polyalkylene glycols (PAGs) have higher volatility than the former lubricants but their oxidation stability is similar or only slightly worse than that of polyolesters. These trends can change depending on the branching and the molar mass of these base oils [7]. On the other hand, the thermal stability of a compound, which can be determined from the maximum temperature of operation, can change depending on the use and conditions that this compound is submitted to. For example, a compound with low oxidation stability and relative evaporation capacity, which was firstly classified as slightly stable, might be used without problems as a lubricant, under an inert atmosphere and in a closed system. Due to these dependences and possibilities, the definition of thermal stability and maximum operation temperature for ILs is an open question [5, 8-13]. Additionally,

thermal

stability

of

ILs

is

currently being

evaluated

using

thermogravimetric (TG) analysis at a single linear heating rate in controlled atmosphere. The on setand peak decomposition temperatures obtained from a single temperature-

3

ramp experiment often overestimates the long-term thermal stabilities of ILs because of the scanning nature of this test [5, 9, 10, 14]. Fast TG analysis scans under a protective atmosphere do not imply long-term thermal stability under these temperatures [14, 15], because isothermal studies at temperatures significantly lower than those indicated by the peak or on setdecomposition have shown that ILs exhibit appreciable decomposition [16]. Thus, in order to stablish a more realistic thermal degradation temperature, some parameters have been defined in the literature; for instance the temperature t0.01/10h, (temperature at which the decomposition of ILs reaches 1% for 10 hours) [17]. This work presents a comparison of the long-term thermal stabilities of three ionic liquids, with the aim of being proposed as lubricants or additives, and five synthetic lubricant bases, using thermogravimetric analysis in dynamic and isothermal modes in air atmosphere. In order to detect the influence of the ion type on thermal stability, two of the three ILs, which were selected due to their potential use as lubricants, have a common anion tris(pentafluoroethyl) trifluorophosphate [(C2F5)3PF3]- (also known as [FAP]- ) and also two of them have a common cation 1-(2-methoxyethyl)-1methylpyrrolidium, [C1OC2C1Pyrr]+. In particular, the ILs studied are: 1-(2-methoxyethyl)-1methyl-pyrrolidinium [C1OC2C1Pyrr][(C2F5)3PF3],

tris(pentafluoroethyl)trifluorophosphate, 1-(2-methoxyethyl)-1-methyl-pyrrolidinium

bis(trifluoromethylsulfonyl)imide,

[C1OC2C1Pyrr][NTf2 ],

and

trihexyl(tetradecyl)phosphonium

tris(pentafluoroethyl)trifluorophosphate,

[P6,6,6,14][(C2F5)3PF3]. The results obtained for these ILs and trends of the thermal stability with the anion and cation structures obtained with other ILs previously studied in our laboratory [14, 15] are presented. Furthermore, as one of the potential applications of these compounds is lubrication, five lubricant bases, a dipentaerythritol hexaheptanoate, DiPEC7, a polypropyleneglycol dimethyleter, PAG2 and three perfluoroalkylpolyethers, Krytox GPL 103, 104 and 105, were also studied in same conditions. Due to their high oxygen content and inherent polarity, PAG lubricants are one of the most versatile types of synthetic lubricants [18-21]. Polyolesters lubricants have applications as high-performance lubricants due to their high stability to thermooxidation, high flash point, and low volatility [22, 23]. Perfluoropolyethers are widely used as topical lubricants on rigid magnetic media to provide a low wear interface [24] and wide application in coatings, paints and adhesives because of their excellent properties, such as water and oil repellent characteristics, thermal, chemical, aging and weather resistances [25]. 4

This work complements the database of thermophysical properties of the selected ILs and lubricant bases previously determined by our research group. Fandiño et al. [26] and Paredes et al. [22, 27] measured the densities and viscosities of DiPEC7 and PAG2 in a broad range of temperatures and pressures and Gaciño et al. [28] measured the densities

and

viscosities

[C1OC2C1Pyrr][(C2F5)3PF3],

of

the

three

ILs

[C1OC2C1Pyrr][NTf2 ]

and

studied

in

this

work,

[P6,6,6,14][(C2F5)3PF3]

at

atmospheric pressure in a broad density range. Viscosities of the three ILs in broad ranges of temperature and pressure were measured by Gaciño et al. [29, 30] and Regueira et al. [31]. Furthermore, the influence of pressure and temperature on densities and the isothermal compressibilities for the three ILs were also reported by Regueira et al. [32, 33] and Gaciño et al. [34]. The results from these works indicated that the ILs containing the [(C2F5)3PF3]- anion the densest and the most viscous, falling in the range of the applications of hydraulic fluids or lubricants. Additionally, the tribological behaviour of Krytox GPL oils and [P6,6,6,14][(C2F5)3PF3] as neat lubricant and as lubricant additive for a steel/steel contact were reported by Otero et al. [1, 35] observing better friction coefficients with ILs than with the reference oils. Additional information is added in this work with the calculation of the maximum temperature at which these fluids can be used, to warranty the correct operation of the lubricants. 2. Experimental The samples of the three selected ILs were kindly provided by Merck KGaA with specified fraction purity higher than 0.97; PAG2 and DiPEC7 were provided by CrodaUniqema and the Krytox GPL oils by Brugarolas. Chemical structure, molecular formula, mass fraction purity and provenance are listed in Table 1, as well as the mean number of the repetitive units of PAG2 and the Krytox GPL oils, which are polymers. ILs were used without further purification, because, in many potential industrial applications, such as lubricants, contact with air do not allow to avoid the residual water content, which is the main impurity. Nevertheless, and taking into account that the influence of water content on thermal stability has not been deeply studied, some thermal properties of ILs purified and saturated of water have been also measured. DiPEC7, PAG2 and Krytox samples are aliquots of those used in previous papers [1, 18, 22, 26, 27, 35], where more information on their characteristics can be found.

5

Table 1. Ionic liquids and lubricant bases used in this work. Name

Abbreviation CAS number

Chemical structure

Mass fraction purity

[C 1OC 2C1Pyrr][NTf 2] 757240-24-7

>0.993a

Merk KGaA

1-(2-methoxyethyl)-1-methyl-pyrrolidinium tris(pentafluoroethyl)trifluorophosphate

[C1OC2C1Pyrr][(C 2F5)3 PF3] 1195983-48-2

>0.990b

Merk KGaA

Trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate

[P6,6,6,14][(C 2F5)3 PF3] 883860-35-3

>0.99c

Merk KGaA

dipentaerythritol hexaheptanoate

DiPEC7 76939-66-7

>95 [18, 22, 26, 27]

Croda

polypropyleneglycol dimethyleter

PAG2

See references [18, 22, 26, 27]

Croda

Perfluoropolyalkylethers

Krytox GPL 103 (n=13) Krytox GPL 104 (n=18) Krytox GPL 105 (n=30)

See reference [36]

1-(2-methoxyethyl)-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide

where n = 13, 18, 30

Impurities are measured by suppliers through: aalkalimetry after ionic exchange that the purity, bElectrophoresis, cNuclear magnetic resonance spectroscopy

6

Brugarolas Made by DuPont

A thermogravimetric analyser (TGA7- Perkin Elmer) operating in dynamic and isothermal modes under dry air atmosphere was used to perform thermal stability studies. Air atmosphere was chosen because it is more restrictive than N2, especially in dynamic studies [14, 15]. Besides, it is more appropriate in order to estimate the maximum operation/service temperature in lubrication applications because in most of them the lubricants are exposed to air. Samples of 3-5 mg were placed in an open platinum pan. Dynamic experiments were performed at temperatures from (100 to 800) ºC with a heating rate of 10 ºC · min-1 and a purge gas flow of 20 cm3 · min-1. The on setand end set temperatures determination procedure is described in previous articles [6, 14, 15]. Temperature at which the (1, 2, 5, 10)% of mass loss appears (t1%, t2%, t5% and t10%, respectively), the remaining mass at on settemperature (Wonset), as well as the temperature of the minimum of the DTG peaks (t1st and t2nd ) were determined from the thermogravimetric curves, TG, and their derivative curves, DTG. Furthermore, isothermal TG analysis was used to determine the long-term thermal stability of ILs. This analysis provides rates of decomposition at a given temperature after a given isothermal-heating time. Seven temperatures lower than ton set were selected for these scans. Taking into account that at 200 ºC the mass loss was lower than 1% in 5 hours for the studied ILs, isothermal experiments at temperatures lower than 200 ºC were not performed for the selected ILs, whereas for the lubricant base oils isothermal assays down to 140 ºC were performed. All liquids were heated to the desired temperature by applying a constant heating rate of 40 ºC · min-1 and the mass loss corresponding to this heating step was not taking into account in case of it was observed, i.e. a correction in sample mass was performed to assign the total mass of the sample to the beginning of isothermal part. A simultaneous DSC/TGA (DSC/TGA1 Mettler Toledo) was also used to characterize the thermal behaviour of these compounds. This device provides trustworthy results using a TGA balance with a complementary DSC heat flow sensor using the same sample for both techniques. This combination allows determining the kind of peak (endothermic or exothermic) associated to the mass loss in order to know the process kind: evaporation or degradation. Experiments were performed under N2 and air atmosphere, at a heating rate of 5 oC · min-1 in a temperature range between (50 and 600) oC. Following the instructions of manufacturers, calibration temperature was

7

performed with melting temperatures of indium and aluminium and the indium enthalpy of fusion was used to heat flow calibration.

3. Results and discussion 3.1 Short-term thermal stability TG and DTG curves of the three ILs and the five lubricant bases are plotted in Figure 1. The mass loss reaches almost 100% at 600 ºC for all the studied compounds, which means that no char is formed during the heating. Three different behaviours can be observed from the comparison of the DTG curves shapes: a) [C1OC2C1Pyrr][NTf2 ], PAG2, Krytox GPL 103, Krytox GPL 104 and Krytox GPL 105 present clearly a unique degradation step, b) [P6,6,6,14][(C2F5)3PF3] shows two resolved peaks corresponding to two different degradation processes in the sample, and c) [C1OC2C1Pyrr][(C2F5)3PF3] and DiPEC7 exhibit one resolved peak followed by a shoulder. In the case of ILs, these thermal behaviours agree with previous results of our research group [14, 15] and Zhou et al. [37] for other ILs with the [NTf2 ]- anion showing one step degradation, and for [(C2F5)3PF3]- ILs presenting two-steps.

a)

8

c)

b)

Figure 1. TG (a) and DTG (b, c) curves of ILs and lubricants: (●) [C1OC2C1Pyrr][NTf2 ], (□) [P6,6,6,14][(C2F5)3PF3], (
) [C1OC2C1Pyrr][(C2F5)3PF3], () DiPEC7, () Krytox GPL 105, () Krytox GPL 104, () Krytox GPL 103, (■) PAG2

Table 2 presents the tonset, tend set, Wonset, and t10% values obtained from the dynamic TG curves, and t1st and t2nd temperatures from DTG curves, for the three ILs and the five lubricants. Up to our knowledge, there are a few previous published studies on the thermal stability for the selected ILs; Zhou et al. [37] have analysed the thermal stability of [C1OC2C1Pyrr][NTf2] with the same heating rate as us but with N2 gas and obtaining an on setvalue of 416 ºC, 5 degrees higher than the corresponding to our value. Also, Reiter et al. [38] have reported on set temperatures for this IL at a heating rate of 5 ºC · min-1 of (345 and 306) ºC under N2 and O2 atmospheres, respectively. Ferreira et al. [39] have reported ton

set and

t1st values (343 and 367 ºC, respectively) for

[P6,6,6,14][(C2F5)3PF3], which are in relative agreement with our values because these authors used 2 ºC · min-1 and, as it is well known [6], degradation temperatures increase with the heating rate of the dynamic TG-DTG studies. The observed trend for the ton

set values

is the following [C1OC2C1Pyrr][NTf2 ] >

[P6,6,6,14][(C2F5)3PF3] > [C1OC2C1Pyrr][(C2F5)3PF3] > DiPEC7 > Krytox GPL-105 > Krytox GPL-104 > PAG2 >Krytox GPL-103. This trend is also observed for t10% and t1st. Hence, the three ILs have higher short-term thermal stability than the selected lubricant bases.

9

Table 2. Thermal results, on settemperature (tonset) and end set temperature (tend

set),

temperature at the 10% of mass loss (t10%), mass loss at on settemperature (Wonset) and temperatures of the minimums of the DTG peaks (t1st and t2nd), from the dynamic scans in air atmosphere measured under atmospheric pressure, (980 ± 10) hPa, and relative humidity of (85 ± 10)% except for DiPEC7 and PAG2 that were measured under relative humidity of 44%. IL

ton set

tend set

t10%

/( C)

/(ºC)

o

/( C)

/(%)

/( C)

/(oC)

411

460

399

82

443

---

[C1OC2C1Pyrr][(C2F5)3PF3] 352

419

349

88

384

417

[P6,6,6,14][(C2F5)3PF3]

363

451

356

88

396

445

DiPEC7

318

445

307

86

370

420

PAG2

212

240

197

76

234

---

Krytox GPL 105

298

373

270

79

352

---

Krytox GPL 104

235

321

228

87

288

---

Krytox GPL 103

200

267

186

83

244

---

o

[C1OC2C1Pyrr][NTf2 ]

Won set

t1st o

t2nd

Expanded uncertainties are U(t) = 6 ºC and U(W) = 2% (0.95 level of confidence (k=2))

With regard to the lubricant bases, Boyde and Randles [23] obtained that degradation of esters as DiPEs is detectable from 275 ºC, which is in good concordance with that here obtained for DiPEC7. Besides, Greaves [20] indicates that PAGs begin to degrade at about 170 °C in the absence of antioxidants, although the inclusion of an antioxidant can improve the thermal stability by (40–70) °C. As concern to Krytox lubricants, the trend obtained is coherent with the volatility values and the estimated useful maximum operating temperatures reported from manufacturer’s reference literature [36]. Due to the important influence of the experimental conditions on the results, that can explain the differences between results of different authors [6, 40], to perform a trustworthy comparison with bibliographic data is difficult. During last five years, continuous studies of thermal stability of ILs and other reference fluids have been performed using the same equipment and conditions [6, 11, 13-15], providing the possibility to stablish a trend of short-term thermal stability with the type of cation, anion and their alkyl chain lengths. Figure 2 shows the values of on-set temperatures of 20 ILs and the 5 lubricant bases determined under dry air atmosphere and with scanning

10

rate of 10 ºC · min-1. It can be clearly seen that the anion influence on the thermal stability of ILs is higher than the cation one, being ILs with [NTf2 ]- and [OTf]- anions the most stable meanwhile [C2C2PO4]- and [C6SO4]- ILs were the least stable, agreeing with the main observations for thermal stability of ILs in literature [35, 41].

Figure 2. On set temperature values for ILs measured at the same known conditions (air atmosphere, heating rate of 10 ºC · min-1 and mass sample of 5±1 mg). [C4C1C1Im][OTf], [C4C1C1Im][NTf2], [C4C1C1Im][(C2F5)3PF3], [C1C1Im][DMP], [C2C1Im][C6SO4], Salgado et al. [14]. [C4C1Pyrr][NTf2 ], [C4C1Pyrr][OTf], [C4C1Pyrr][B(CN)4], [C4C1Pyrr][(C2F5)3PF3], [C4 C1Pyrr][(C4F9)3PF3], Salgado et al. [15]. [C2py][NTf2], [Chol][NTf2], [C2C1Im][OTf], [C2py][OTf], [C2C1Im][BETI], Villanueva et al. [11]. [P4,4,4,2][C2C2PO4] Otero et al. [35]. Different colours show the ILs with a common anion and the base lubricants studied are in red colour. Although cation influence is lower than that of the anion [14, 15, 42-44], results show that the sequence with cation family is: Imidazolium > Pyrrolidinium > Piridinium > Choline According to Figure 2 the highest long chain corresponds with the lowest thermal stability. Thus, Zhou et al. [37] have found that replacing the butyl group in the cation with a methoxyethyl group leads to a decrease in the thermal stability for ILs with the anions [CnF2n+1BF3]- (for n=0 to 4) and [NTf2]-. This fact completely agrees with results of Salgado et al. [15] for [C4C1Pyrr][(C2F5)3PF3] and [C4C1Pyrr][NTf2], with ton

11

set of

(358 and 417) ºC respectively, and with those presented in this work for [C1OC2C1Pyrr][(C2F5)3PF3] and [C1OC2C1Pyrr][NTf2], with ton setof (352 and 411) ºC. Also, most of the ILs have higher short-term thermal stability than the selected lubricant bases. As it can be seen in Table 2, ton setdata are higher than t10% values, meaning that mass lost at on set temperatures are higher than 10%, and therefore they are not an optimum parameter to characterize the maximum operation temperature of a compound, as it has been indicated by other authors [6, 8, 9, 26, 30-35], and deeper studies, such as isothermal scans, are needed, as it can be seen further on.

3.2. Water effect on short-term thermal stability of ILs Frequently, bibliographic references indicate the important influence of impurities, such as water, on the thermophysical properties of ILs; nevertheless scarce data about the changes in thermal stability results as consequence of impurities can be found. With the aim to provide new information to this controversial question, a comparison between the TG-dynamic scans of three ILs was performed in two different conditions of water content (as supplied and saturated). Saturated samples were obtained keeping in an open bottle the selected ILs under atmospheric conditions (temperature (16 ± 2) ºC, pressure (987 ± 5) hPa, and relative humidity of (82 ± 7)%) up to reach a constant mass (10 days approximately), weighting samples every 24 hours in a Sartorius balance with a precision of 0.00001 g. Water contents of the pure and water-saturated samples were measured with a Karl-Fischer coulometric titrator (Metler Toledo DL32) and results are shown in Table 3. As it can be seen, [C1OC2C1Pyrr][NTf2] has a high capacity for absorbing water, reaching a water content value that is around twenty times that of the pure sample. ILs with anion [(C2F5)3PF3]- present the lowest values of water content after saturation, especially for [P6,6,6,14][(C2F5)3PF3] which shows a relatively hydrophobic character with a water saturation content of 0.026%. The comparison between TG and DTG curves of [C1OC2C1Pyrr][NTf2] and [C1OC2C1Pyrr][(C2F5)3 PF3] before and after water saturation is presented in supplementary material, as Figure S1. These ILs, which are those with the highest difference in water content between supply and saturated conditions, presented similar TG and DTG curves for both water contents. The main differences are shown at 100 ºC, where the weight percentage of saturated [C1OC2C1Pyrr][NTf2] is 12

slightly lower than the corresponding to dry IL, approximately 0.4%, which matches with the water absorbed and released in TG experiments at this temperature. Additionally, on set temperatures for the three ILs, in both conditions, are also presented in Table 3. Results show a little decrease in this parameter after the saturation process, nevertheless differences between water saturation and supply conditions are lower than the expanded uncertainties of the apparatus. These results agree with Valkenburg et al. [45], who studied the effect of water and chloride contamination on the thermal stability of three imidazolium ILs, concluding that both did not affect this parameter, only a weight loss at about 100 ºC due to water evaporation was observed, but the remaining IL decomposes at the same temperatures than the corresponding dry ILs. Additionally, Huddleston et al. [46] have found different responses after drying some imidazolium based ILs, with a slight decrease in ton set from (360 to 349) ºC after drying, as in the case of [C4C1im][PF6]; an increase in ton set from (390 to 417) ºC after drying for [C6C1im][PF6], and similar results of this characteristic temperature for [C8C1im][PF6] with two very different water contents, of (6666 and 388)10-6. Thus, despite the insignificant influence of the water content on the thermal stability of the ILs here studied, the role of water on this property cannot be generalized to all ILs and additional studies must be performed to identify the water content-thermal stability relationship.

Table 3. Water contents and degradation on set temperatures (ton set) for the ILs corresponding to water saturation content and to supply conditions measured under atmospheric pressure, (992 ± 5) hPa, and relative humidity of (80 ± 5)%. IL [C1OC2C1Pyrr][NTf2] [C1OC2C1Pyrr][(C2F5)3PF3] [P6,6,6,14][(C2F5)3PF3]

Water content 10-6 As supplied saturated 174 3700 139 1028 193 264

ton set/(ºC) As supplied saturated 411 406 352 349 363 358

Expanded uncertainties (k=2) are U(t) = 6 ºC.

3.3. Isothermal study As it was pointed out at the end of section 3.1, to make a more realistic description of the thermal stability and in order to perform a more complete comparison, the three ionic liquids and the two synthetic lubricant bases with the highest ton

set

(namely

[C1OC2C1Pyrr][NTf2], [C1OC2C1Pyrr][(C2F 5)3PF3], [P 6,6,6,14][(C2F5)3PF3], DiPEC7 and 13

Krytox GPL 105) were subjected to several isothermal experiments at different temperatures. Figure 3 shows the isothermal TG curves of these three ILs and the two lubricant bases, as a function of time, for the different selected exposition temperatures. The first observation is, as it was expected, a fast degradation at higher temperatures, which are close to tonset.

14

Figure 3. Isothermal scans of the selected ILs and lubricants: (a) [C1OC2C1Pyrr][NTf2], (b) [P6,6,6,14][(C2F5)3PF3], (c) [C1OC2C1Pyrr][(C2F5)3PF3], (d) DiPEC7, (e) Krytox GPL-105. All experiments were performed at atmospheric pressure, (990 ± 10) hPa, and relative humidity of (83 ± 15) %.

15

A comparison of the isothermal scans at 260 ºC for the five fluids is presented in Figure S2 in the supplementary material. ILs are much more stable than the lubricants, for the selected experimental conditions, being the higher degradation about 80% after 100 minutes for DiPEC7 and 20% for the IL [P6,6,6,14][(C2F5)3PF3] in the same time. From this figure it can be deduced the following sequence from more to less stability [C1OC2C1Pyrr][NTf2] > [C1OC2C1Pyrr][(C2F5)3PF3] > [P6,6,6,14][(C2F5)3PF3] > Krytox GPL 105 > DiPEC7 being this order similar to the one obtained from dynamic experiments

3.4. Kinetics of isothermal degradation The kinetics of mass loss of the ILs [C1OC2C1Pyrr][NTf2], [C1OC2C1Pyrr][(C2F 5)3PF3], [P6,6,6,14][(C2F5)3PF 3] and the two selected lubricants Krytox GPL 105 and DiPEC7 was analysed from isothermal TGA results following the methodology reported in previous articles [6, 14, 15]. The temperature dependence on the rate of mass loss, k, is represented by the Arrhenius equation:  − Ea   k = A exp  RT 

(1)

where Ea is the activation energy and A is the pre-exponential coefficient. This relation allows the prediction of the decomposition rate at any temperature. Figure S3 in supplementary material shows the relation between the values of ln k and T-1.Table 4 summarizes the activation energy of the degradation process and the pre-exponential coefficients obtained from the linear fitting corresponding to the fluids studied in this work, as well as the results for other ILs determined in previous studies [11, 13-15] using the same experimental conditions.

Table 4. Pre-exponential coefficients, A, and activation energies, Ea, for ILs and lubricants (ordered from highest to lowest activation energy) obtained from the Arrhenius equation (1). IL

Ea / (kJ · mol-1)

A / min-1

[C2py][OTf] [11]

185

2.08 1016

[Chol][NTf2] [11]

169

2.43 1014

[C2C1Im][OTf] [11]

161

1.58 1013

[C4C1Pyrr][(C2F5)3PF3] [15]

153

1.05 1016

16

[C4C1C1Im][OTf] [14]

148

1.10 1012

[C4C1Pyrr][NTf2] [15]

147

1.56 1012

[C2py][NTf2] [11]

143

9.37 1011

[C2py][C1SO 3] [11]

142

4.91 1013

[C4C1C1Im][(C2F5)3PF3] [14]

139

9.36 1011

[C4C1C1Im][NTf2] [14]

129

1.29 1010

[C1OC2C1Pyrr][NTf2]

126±13

2.41 1010

[C2C1Im][BETI] [11]

109

2.42 109

[P6,6,6,14][(C2F5)3PF3]

104±11

1.10 1010

[C1OC2C1Pyrr][(C2F5)3PF3]

110±7

3.53 109

Krytox GPL 105

92±3

1.31 109

DiPEC7

88±17

1.14 109

As it can be seen, these activation energy values ranged between 185 kJ · mol-1 for [C2py][OTf] and 88 kJ · mol-1 for DiPEC7, and it is truly interesting that all the ILs presented higher values of activation energy (Ea>100 kJ · mol-1) than the base lubricants (Ea<100 kJ · mol-1). Up to our knowledge, no values of this parameter for the studied fluids have been previously studied by other authors. In case of the selected ILs, the values obtained for Ea are in concordance with those reported in the literature for other ILs [14, 16, 45, 47-50]. As it can be seen, the preexponential factors differ in several orders of magnitude (from 10 16 to 108). The preexponential factor (also named frequency factor) is related to the frequency of molecular collisions occurring, whether or not the collision leads to a reaction [51]. The different A values suggest that significantly fewer collisions are occurring for [C1OC2C1Pyrr][(C2F5)3PF3] than for [C1OC2C1Pyrr][NTf2], although the lower Ea value for [C1OC2C1Pyrr][(C2F5)3PF3] indicates that a greater proportion of collisions will result in a decomposition reaction. These different A values may be a result of the lower mobility of the [(C2F5)3PF 3]- anion with respect to the [NTf2]-, which is highly flexible and explores a number of conformations [52]. This is also the reason why the [NTf2] ILs are less viscous than other ILs like those containing the anion [(C2F 5)3PF3]- [28, 52, 53].

3.5. Maximum operation temperature Up to now, a clear criterion does not exist on the degradation level allowed in different applications, finding in literature a wide range, from 1% in one year [9] to 10% in 10 h

17

[4]. Indeed, one of the main reasons is that the criterion should depend on the application. In this work, a comparison between the maximum operation temperatures (MOT) determined using different criteria and different methods is presented. Firstly, as in a previous article [15], four degradation levels, (1, 2, 5 and 10%) at different temperatures were selected in order to obtain information about how much time an IL takes to degrade each level. From isothermal scans, the time that each IL takes to decompose in the above percentages was determined and, from these data, a correlation with a decreasing exponential function of the temperature (equation 2) was obtained, being t’ the time in minutes, T the temperature in K, and, B and C the fitting coefficients.

t ' = B e -C(T - 273)

(2)

The corresponding fitting parameters are presented in Table S1 (supplementary materials). Taking into account that depending on intended application different appropriate degrees of degradation and time periods could be chosen, the MOT at which an IL could be used can be estimated from these fits. Table 5 shows the MOT temperatures of the selected fluids, calculated as the temperature that each degradation level takes place in 10 h. The stricter the criterion, the lower the MOT is. Taking into account that in all of these calculations an important loss of accuracy can appear, (1015)%, these temperatures should not be overcome in any case to asses these criteria. Figure 4 presents, as examples, the experimental data from isothermal scans for the lowest degradation level (1%) and the corresponding exponential fittings of the IL [P6,6,6,14][(C2F5)3PF 3] and the lubricant base Krytox GPL 105; MOT temperatures corresponding to a mass loss of 1% in 10 h were also indicated in this figure.

18

Figure 4. Experimental data, fitting curve and MOT temperature corresponding to the degradation level of 1% in 10 h for the IL [P6,6,6,14][(C2F5)3PF3] (a) and the lubricant base Krytox GPL 105 (b).

Table 5. MOT temperatures for different degradation levels (1%, 2% 5% and 10% in 10h). t0.01/10h/ ºC

t0.02/10h/ ºC

t0.05/10h/ ºC

t0.10/10h/ ºC

[C1OC2C1Pyrr][NTf2 ]

201

206

223

252

[C1OC2C1Pyrr][(C2F5)3PF3]

143

145

185

219

[P6,6,6,14][(C2F5)3PF3]

157

172

196

212

DiPEC7

111

128

---

---

Krytox GPL 105

60

87

129

172

19

Some authors suggested the application of other methods, based in dynamic and isothermal TGA scans, as could be the following: - Wooster et al. [17] and Baranyai et al. [54] suggested that the temperature at which 1% degradation occurs in 10 h (t’0.01/10 h) is a good indicator of thermal stability. Furthermore, Wooster et al. [17] established a method to estimate T0.01/10 h, expressed in Kelvin, from dynamic scans by using the equation:
. ⁄ = 0.82
 ⁄   3 being T(dW/dt)≠0 the temperature in Kelvin at which the first appreciable weight loss occurs. Following this methodology, the estimation of maximum temperature was done and results are presented in Table 6. Efimova et al. [55] followed the criterion of Seeberger et al. [9], who took into account isothermal conditions, a level of degradation of 1% and the use of an IL in technical applications in closed systems, where the mass loss by evaporation is irrelevant and only thermal degradation has to be considered. They proposed the use of Eq. 4 to calculate the MOT for a defined time of operation (tmax) 
=

 / 4.6 + ln  ·

!" 

4

Thereby, the temperature decreases systematically with higher period of exposition. Hence, following this criterion, the MOT for the IL [C1OC2C1Pyrr][NTf2] changes from 192 ºC for 1 h, 151 ºC for 1 day and to 91 ºC for 1 year (8000 h) [9]; whereas for both base oils it does not exceed 100 ºC in any case, as it can be seen in Figure 5, which relates the MOT (in Celsius degrees) with the operating time (in hours) for the five fluids studied in this work. Surprisingly, these results show that it is impossible to use both lubricant bases along a year, which is in contradiction with the technical specifications of these oils. This fact could be explained by the assumption of an irrelevant evaporation, which might not be applicable to these base oils.

20

Figure 5. Calculation of the maximum operation temperature (MOT) of the selected fluids

((■)

[C 1OC2C1Pyrr][NTf2],

(▲)

[C1OC2C1Pyrr][(C2F5)3PF3],

(X )

[P6,6,6,14][(C2F5)3PF 3], (□) Krytox GPL 105, (○) DiPEC7) depending on the operating time, according to Eq. 3 [55].

Table 6 summarizes the t 0.01/10h values (expressed in Celsius) predicted by the three methods used here. The highest values of MOT correspond to the dynamic method used by Wooster et al. [17], this fact was also observed in a previous work [15]. The lowest values of MOT are that obtained from Efimova et al. [55] and Seeberger et al. [9] method. Up to our knowledge no previous data have been published about this parameter for the selected compounds. The only comparison that can be done is with Krytox GPL 105, whose technical information, supplied by manufacturer [36], indicates 1% of mass loss at 66 ºC during 22 h which is in very good agreement with the value of 65ºC obtained after the adaptation of Efimova et al. [55] method to 22 h of exposition. We should remark that the sample of Krytox GPL 105 used in this work is not exactly the same as that Krytox GPL 105(H-1), which appear now in the link of reference [36].

Taking into account the data obtained from the isothermal study, the observed trend, from higher to lower thermal stability, is the following [C1OC2C1Pyrr][NTf2] > [C1OC2C1Pyrr][(C2F5)3PF3] > [P6,6,6,14][(C2F5)3PF3] > Krytox GPL 105 ≈ DiPEC7 that is the same as those obtained with the Ea and ton set values. It can be seen again that ILs

21

with [NTf2]- anions show higher values of thermal stability [11, 13-15], whatever the parameter studied and the criterion used are.

Table 6. Comparison between temperatures (in ºC) values corresponding to the mass loss of 1% during 10 h obtained from isothermal studies and those estimated according to Wooster et al. [17] and Efimova et al. [55].

t'0.01/10h /ºC Isothermal study [C1OC2C1Pyrr][NTf2 ] 201 [C1OC2C1Pyrr][(C2F5)3PF3] 143 [P6,6,6,14][(C2F5)3PF3] 157 DiPEC7 112 Krytox GPL 105 60

t0.01/10h /ºC Dynamic study [17] 258 180 196 198 125

t0.01/10h /ºC [55] 161 128 151 59 73

3.6. Trying to discern between evaporation and degradation As it was previously pointed out in this paper, one of the most discussed open questions is the mechanism associated to the mass loss. In order to make progress on this topic and discern in an easy way if degradation or evaporation (or both) took place at high temperatures, TGA experiences were done using small quantities of sample. Firstly, the methodology of Götz et al. [56] was followed, who suggested that it is possible to distinguish between evaporation or degradation visually if colour change is observed after the heating. This procedure was done for the three ILs and one of the lubricants, Krytox GPL 105, in air atmosphere. Thus, these fluids were maintained in isothermal conditions in a TGA device at temperatures lower than ton set until the mass loss was 10 %. Afterwards, the sample pan was removed from the TG and the colour change

was

evaluated.

The

appearance

of

the

ILs

[C1OC2C1Pyrr][NTf2],

[P6,6,6,14][(C2F5)3PF3] and [C1OC2C1Pyrr][(C2F5)3PF3] changed after the exposition (< 20 min) at (360, 320 and 340) ºC respectively, from a colourless liquid to a dark brown, especially for the two first ILs, indicating decomposition of these ILs in these conditions. Meanwhile, the lubricant maintained his transparent appearance after the isothermal TG scan at 260 ºC, meaning that the mass loss was probably mainly due to evaporation. A comparison of pictures taken before and after a mass loss of 10% under different isothermal thermogravimetric scans is shown in Figure 6.

22

Furthermore we have chosen the IL [C1OC2C1Pyrr][(C2F5)3PF3], which presented the less dark colour after isothermal scan, to complete this visual study. This IL was subjected to seven TG scans at 320 ºC during different times to obtain seven degradation levels (1 -80) %. As result it was observed that the greater the mass loss, the darker the sample colour is, concluding that the mass loss of this IL in air atmosphere is mainly due to decomposition. Figure S4, in supplementary materials, collects pictures of the sample during the different parts of this study.

[C1OC2C1Pyrr] [NTf2]

before

[P6,6,6,14] [(C2F5)3PF3]

after

before

[C1OC2C1Pyrr] [(C2F5)3PF3]

before

After

Krytox GPL 105

after

before

After

Figure 6. Colour change of ILs and Krytox GPL 105 lubricant for a mass loss of 10% for different isothermal scans. Isothermal TGA of 360 ºC for [C1OC2C1Pyrr][NTf2 ], 320 ºC for [P6,6,6,14][(C2F5)3PF3], 340 ºC for [C1OC2C1Pyrr][(C2F5)3PF3] and 260 ºC for Krytox GPL 105. (For interpretation of the colour, the reader is referred to the web version of this article.)

On the other hand, to complete this goal, besides the visual degradation a comparison between heating experiments under inert and air atmospheres in simultaneous DSC/TGA of some of the selected fluids (a lubricant base and two ILs) was performed. It must be taken into account that the presence of oxygen gas in air atmosphere introduces the possibility of oxidation or combustion during TG scans, 23

which does not happen in inert atmosphere [51]. Thus, no change in the thermal stability parameters in both atmospheres would suggest, interestingly, that neither oxidation nor combustion were part of the possible thermal decomposition mechanisms. Furthermore, the endothermic or exothermic character of the DSC peaks, associated to the TG mass loss, could also provide additional information about these mechanisms (evaporation is always an endothermic reaction and combustion is mainly an exothermic reaction). Figure 7.a presents the TG signal obtained from simultaneous DSC-TGA of the lubricant PAG2 in air and N2 atmospheres. These curves presented similar shape although a significant shift to higher temperatures (approximately 90 ºC) can be observed when the atmosphere changes from air to nitrogen, indicating that the mass loss mechanism depends on the working atmosphere. Figure 7.b shows the DSC signal of this base lubricant in both atmospheres. This big exothermic peak observed in air atmosphere is compatible with combustion, i.e. fluid degradation, and the small endothermic peak observed in nitrogen atmosphere is compatible with evaporation. It is important to indicate that the sensibility of the simultaneous DSC signal has not been sufficient to identify this small peak; for this reason similar experiences (atmosphere, heating rate and temperature interval) were performed in the DSC Q-100 (TAInstrument) used for the analysis of transitions in previous works [11], which is much more sensitive and allows the identification of the weak energetic processes.

Figure 7. (a) TG signal obtained from simultaneous DSC-TGA and (b) DSC curves (Exo down) of PAG2 in air (dashed line) and nitrogen (solid line) atmosphere.

With regards to the behaviour of the ILs, which can be seen in Figure 8 and Figure 9, TG curves of [P6,6,6,14][(C2F5)3PF3] and [C1OC2C1Pyrr][(C2F 5)3PF3] show that both ILs present higher thermal stability in nitrogen than in air atmospheres, also 24

observed in previous studies performed in our laboratories [14]; nevertheless, these differences between atmospheres are lower than the obtained for the lubricant base. As it was pointed out in section 3.1., some steps (or shoulders) can be observed in TG curves of both ILs, indicating that more than one process appeared during their mass loss. Each detected TG step has a corresponding peak in DSC profiles, but in this case, all the peaks are exothermic, in both atmospheres, although the intensity of the peaks observed in air atmosphere (also compatible with combustion) are higher than those detected in nitrogen atmosphere and then incompatible with evaporation, being these results in agreement with the previous visual observations for these ILs.

Figure 8. (a) TG signal obtained from simultaneous DSC-TGA and (b) DSC curves (Exo down) of [P6,6,6,14][(C2F5)3PF3] in air (dashed line) and nitrogen (solid line) atmosphere.

Figure 9. (a) TG signal obtained from simultaneous DSC-TGA and (b) DSC curves (Exo down) of [C1OC2C1Pyrr][(C2F5) 3PF3] in air (dashed line) and Nitrogen (solid line) atmosphere.

As a result of all these findings, the working atmosphere has a decisive role on the mechanisms of TG mass loss of the fluids, being the combustion the most probable 25

mechanism involved in the mass loss produced in air atmosphere for base oil and ILs and the evaporation was only possible in inert atmosphere for the studied base lubricant.

4. Conclusions Thermal stability of three ionic liquids (ILs) and five lubricant bases (a PAG, DiPEC7, and three perfluoroalkylpolyethers), was determined by thermogravimetric analysis. According to on set temperatures in air atmosphere, the thermal stability decreases following

the

sequence:

[C1OC2C1Pyrr][NTf2]

>

[P 6,6,6,14][(C2F 5)3PF3]

≈

[C1OC2C1Pyrr][(C2F5)3PF3] > DiPEC7 > Krytox GPL 105 > Krytox GPL 104 > PAG2 > Krytox GPL 103. Results were compared with the obtained, using the same experimental conditions, in previous works, concluding that the anion influence on the thermal stability is higher than the cation, being [NTf2]- and [OTf]- based ILs the most stable. All the ILs here studied showed better stability in air atmosphere than lubricant oils. Besides, the water content has not a clear influence on the thermal stability of the studied ILs.

In order to obtain information about the long term stability in air atmosphere, isothermal studies were performed for the three ILs and the two more stable lubricants and, when comparing at an intermediate temperature, the thermal stability diminishes in the order [C1OC2C1Pyrr][NTf2] > [C1OC2C1Pyrr][(C2F5)3PF3] > [P6,6,6,14][(C2F5)3PF3] > Krytox GPL 105 ≈ DiPEC7, that is almost the same as the above sequence.

The activation energy of thermal degradation process was obtained from kinetic analysis

using

the

Arrhenius

equation.

In

this

case

the

sequence

was

[C1OC2C1Pyrr][NTf2] > [C1OC2C1Pyrr][(C2F5)3PF3] ≈ [P6,6,6,14][(C2F 5)3PF3] > Krytox GPL 105 ≈ DiPEC7, that is similar to that obtained from the ton

setvalues,

being the

highest energy value corresponding to the most stable IL.

Finally, an estimation of the maximum operation temperature of these materials using three different methods was done, selecting the value t0.01/10h as a good indicator for the thermal stability. Values of this parameter were calculated from isothermal studies, and the obtained sequence for the thermal stability was, in this case [C1OC2C1Pyrr][NTf2] >

26

[P6,6,6,14][(C2F5)3PF 3] > [C1OC2C1Pyrr][(C2F5)3PF3] > DiPEC7 > Krytox GPL 105 that is the same than that obtained according to ton set and Ea values.

A very important fact that this work tries to deal with is the knowledge of the mechanism (evaporation or degradation) associated to the mass loss of ILs at high temperatures using small quantities of sample. In this study a developed method of visual degradation over selected compounds completed with DSC/TG studies in air and nitrogen working atmospheres has been applied, concluding that combustion is the most probable mechanism in oxidative atmosphere for the selected ILs and lubricants, and that a different mechanism occurs under nitrogen atmosphere, although evaporation seems to be the main important for lubricant bases.

Acknowledgements Authors acknowledge M. Gómez (RIAIDT-USC) the technical support for DSC-TG measurements. Authors acknowledge Merck KGaA and Brugarolas S.A. for providing us the IL and Krytox amples. This work was supported by Spanish Ministry of Economy and Competitiveness and FEDER Program through the CTQ2011-23925 project as well as by Xunta de Galicia through the EM2013/031 and GRC ED431C 2016/001 projects and the Galician Network of Ionic Liquids (ReGaLIs) R2014/015.

27

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Highlights - Thermal stability in air atmosphere of three ILs and five lubricant bases was studied. - ILs showed better short-term stability in air atmosphere than lubricant oils. -

Similar stability trends according to tonset, activation energy and MOT were obtained. Water content has not a clear influence on the thermal stability of studied ILs. Visual degradation with DSC/TG in air and nitrogen atmospheres has been applied.

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