Synthesis and Tribological Behaviour of 1,3,4-Thiadiazole Schiff Base Derivatives as Multifunctional Lubricant Additives

Shen Qiaohong; Chen Hongbo; Li Lingdong

(School of Petroleum and Chemical Engineering, Dalian University of Technology, Dalian 116024)

Synthesis and Tribological Behaviour of 1,3,4-Thiadiazole Schiff Base Derivatives as Multifunctional Lubricant Additives

Shen Qiaohong; Chen Hongbo; Li Lingdong

(School of Petroleum and Chemical Engineering, Dalian University of Technology, Dalian 116024)

Six new 1,3,4-thiadiazole Schiff base derivatives were synthesized and characterized by IR spectroscopy and1H NMR spectrometry, and their anti-corrosion properties and thermal stability were investigated via thermogravimetric analysis (TGA) and copper strip corrosion test. The tribological behavior of the said Schiff base derivatives was evaluated on an Optimol SRV?4 oscillating reciprocating friction and wear tester. The worn surfaces of the steel discs were investigated using a scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS). The test results indicated that these thiadiazole Schiff base derivatives possessed favourable thermal stability, corrosion inhibiting ability and the capability of improving the tribological characteristic of the base oil effectively. It is assumed that the adsorbed additives probably reacted with the steel surfaces during the friction process, resulting in the formation of a protective flm composed of sulphates, sulphides and organic nitrogen compounds.

1,3,4-thiadiazole; Schiff base; tribological behaviour; corrosion inhibiting ability; lubricant additives

1 Introduction

Lubricating oil is the blood of modern industrial and defence work, and lubricant additives are often used practically to improve the tribological performance of base stock, especially under some rigorous working conditions[1]. However, the traditional lubricant additives, such as zinc dialkyl dithiophosphate (ZDDP), contain heavy metal elements that are believed to produce ash which contributes to particulate matter in automotive exhaust emissions. In addition, the phosphorus content in lubricant additives is usually restricted because phosphorus is thought to limit the service of catalytic converters that are used on cars to reduce pollution[2-4]. Therefore, it is of great signifcance to develop environmentally friendly, energy saving, and ashless lubricant additives for base stocks[5-9].

It has been observed that certain sulphur and nitrogencontaining heterocyclic compounds have certain loadcarrying and lubrication capacity and are used to improve the tribological characteristics of base lubricants[10-25]. Once these heterocyclic compounds are adsorbed by the friction surface in the process of friction, intermolecular hydrogen bonds are readily formed due to the higher electronegativity and smaller atomic radius of the nitrogen atom. This phenomenon can result in lateral gravity enhancement and oil film strength enlargement which contribute to the improvement of the corrosion inhibiting, anti-wear, friction-reducing and extremepressure performance[8,26-29]. 5-Amino-2-mercapto-1,3,4-thiadiazole (AMT) is a common example of these heterocyclic compounds and is often applied in pharmaceutical and agricultural sectors, and oil additives industry[9,28,30]; therefore, derivatives of AMT may be used as effective and multifunctional additives for lubricating oils as well. However, notably few examples of the use of Schiff base derivatives of AMT serving as the frictionreducing and extreme-pressure lubricating oil additives have been reported.

In this paper, six new Schiff base derivatives were synthesized and characterized by1H NMR spectrometry and IR spectroscopy. The tribological behaviour of these derivatives serving as additives in the 150SN base oil was investigated by an SRV?4 oscillating reciprocating friction and wear tester. For closer examination, the lubricating mechanisms and analyses of the worn steelsurfaces were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).

2 Experimental

2.1 Chemicals

Analytical reagent-grade chemicals 5-amino-2-mercapto-1,3,4-thiadiazole, chloroacetic acid, alkyl alcohol, sodium hydroxide, potassium hydroxide, potassium carbonate, salicylaldehyde,p-hydroxybenzaldehyde, ethyl alcohol, acetic acid, acetone, dichloromethane, methanol, ethyl acetate, petroleum ether and deionized water were used to synthesize the additives D1, D2, D3, D4, D5, and D6. The 150SN base oil was used as the base stock, which was produced by the Dalian Petrochemical Company. The performance parameters for 150SN oil are shown in Table 1.

Table 1 The performance parameters of the base oil

2.2 Synthesis

Thiadiazole A (6.5 mmol) was added to a solution of potassium hydroxide (9.1 mmol) in deionized water under stirring until all solids were dissolved. Next, the corresponding chloroacetate (6.5 mmol) in ethanol (5 mL) was slowly added to the clear solution. After stirring for approximately 4 h at room temperature, the reaction mixture was filtered, and the white powder B was collected and dried under vacuum, when the yield of compound B was 88% (R1=n-C4H9), 89% (R1=sec-C4H9) and 92% (R1=n-C6H13), respectively (see Scheme 1). Thiadiazole derivative B (0.01 mol) was dissolved in 20 mL of ethanol prior to the addition of substituted benzaldehyde C (0.01 mol). Acetic acid (0.01 mmol) was added as a catalyst and the solution was heated under refux at 80 °C to give the products D1—D3 after being subjected to reaction for 4 h or the products D4-D5 after being subjected to reaction for 8 h. Evaporation of the solvent resulted in a solid, which was recrystallized in ethanol to obtain the crude product. The final products were purifed by silica gel column chromatography using ethyl acetate/petroleum ether (1:3) as the eluant. To synthesize the product D6,para-hydroxybenzaldehyde (E) was protected using ethyl chloroacetate in the presence of anhydrous potassium carbonate and dry acetone[31]to give the product F at a yield of 81%. D6 was then formed according to the method similar to that for preparing D1—D5, when F reacted on B in ethanol with acetic acid under reflux for 12 h followed by analysis by column chromatography using CH2Cl2/MeOH (25:1) as the eluant.

The structure, yield and physical properties of additives D1—D6 are shown in Table 2; and the IR and1H NMR spectrometric characterization data are shown below.

D1:1H NMR (DMSO-d6, 500 MHz)δ: 11.25 (s, 1H), 9.10 (s, 1H), 7.02-7.90 (m, 4H), 4.30 (s, 2H), 4.11 (t, 2H), 1.55-1.60 (m, 2H), 1.40-1.21 (m, 2H), 0.88 (t, 3H); IR (KBr, cm-1)v: 3059, 2962, 2874, 1732, 1563, 1604, 1460, 1365, 1285, 1250, 1201, 769.

D2:1H NMR (CDCl3, 500 MHz)δ: 11.79 (s, 1H), 9.04 (s, 1H), 7.03-7.49 (m, 4H), 4.90-5.01 (m, 2H), 4.15 (s, 2H), 1.59-1.65 (m, 2H), 1.27 (d, 3H), 0.93 (t, 3H); IR (KBr, cm-1)v: 3058, 2976, 2930, 2878, 1732, 1574, 1605, 1459, 1366, 1277, 1194, 1154, 753.

D3:1H NMR (CDCl3, 500 MHz)δ: 11.80 (s, 1H), 9.06 (s, 1H), 7.53-7.46(m, 2H), 7.18-6.89 (m, 2H), 4.21 (t, 2H), 4.17 (s, 2H), 1.67 (m, 2H), 1.31 (m, 6H), 0.88 (t, 3H); IR (KBr, cm-1)v: 3059, 2959, 2930, 2856, 1736, 1572, 1607, 1497, 1463, 1360, 1280, 1199, 1154, 753, 660.

D4:1H NMR (DMSO-d6, 500 MHz)δ: 10.63 (s, 1H), 8.75 (s, 1H), 7.90 (dd, 2H), 6.94 (dd, 2H), 4.26 (s, 2H), 4.09 (t, 2H), 1.58-1.51 (m, 2H), 1.31 (m, 2H), 0.86 (t, 3H); IR (KBr, cm-1)v: 3448, 3116, 2955, 1735, 1522, 1578, 1458, 1380, 1292, 1173, 1155, 844, 653.

D5:1H NMR (DMSO-d6, 500 MHz)δ: 10.63 (s, 1H), 8.75 (s, 1H), 7.89 (d, 2H), 6.94 (d, 2H), 4.25 (s, 2H), 4.09 (t, 2H), 1.58-1.52 (m, 2H), 1.29-1.21 (m, 6H), 0.82 (t, 3H);IR (KBr, cm-1)v: 3454, 3101, 2956, 2855, 1736, 1520, 1600, 1453, 1381, 1292, 1169, 1123, 843, 654.

D6:1H NMR (DMSO-d6, 500 MHz)δ: 8.84 (s, 1H), 8.01-7.96 (m, 2H), 7.18-7.09 (m,2H), 4.95 (s, 2H), 4.26 (s, 2H), 4.13-4.07 (t, 4H), 1.57-1.54 (m, 4H), 1.23 (m, 12H), 0.83 (t, 6H); IR (KBr, cm-1)v: 3424, 2954, 2928, 2857, 1763, 1725, 1572, 1596, 1517, 1372, 1308, 1196, 1076, 840, 689.

Table 2 Structure and physical properties of D1—D6

Scheme 1 Synthetic routes of the desired additives D1-D6

2.3 Characterization

The base oil (150SN) and additives were mixed thoroughly before the measurements. Briefly, the additives D1—D6 were separately dissolved in 150SN oil to give solutions containing 0.1% of each additive. The thermal behaviour of additives D1—D6 was studied on a Triton DMA/SDTA 861e thermogravimetric analyser (TGA) which was preheated from ambient temperature to 600 °C at a temperature increase rate of 10oC /min in the nitrogen atmosphere. The tribological behaviour of derivatives containing the friction-reducing and extreme-pressure additives in 150SN oil was evaluated on an Optimol SRV?4 oscillating reciprocating friction and weartester. The morphology and chemical composition of the worn surfaces were analysed by a QUANTA FEG 250 scanning electron microscope (SEM) and an energy dispersive X-ray spectroscope (EDS).

2.4 Thermal stability test

The thermal stability of synthetic additives D1-D6 was investigated on a Triton DMA/SDTA861e thermogravimetric analyser. 10 mg of each additive sample were placed in the instrument and the temperature was programmed to increase from 20 °C up to 600 °C at a temperature increase rate of 10 °C/min in the nitrogen atmosphere. The weight and heat flow values of the samples were monitored.

2.5 Copper strip test

Copper corrosion tests were conducted at 100oC /120 °C for 3 h and at 100oC /120 °C for 10 h according to the national standard test method GB/T 5096—1985, which was similar to ASTM D130. The polished copper strip and a suitable vessel containing the sample oil were used. A piece of bright fnished copper strip was immersed into 30 mL of the base oil or the base oil/additive blend and heated at 100oC /120oC for 3 h and at 100oC /120oC for 10 h. At the end of the test cycle, the copper strip was washed and compared with the corrosion standard tint board.

2.6 SRV friction and wear tests

The Optimol SRV?4 oscillating reciprocating friction and wear tests were performed at 50oC. Contact between the frictional pairs was achieved by pressing the upper running ball (with a diameter of 10 mm, made of AISI 52100 steel) with a hardness of approximately 59—61 HRC against the lower stationary disc (?24×7.9 mm, made of AISI 52100 steel) with a hardness of approximately 63—67 HRC at a frequency of 50 Hz, a sliding amplitude of 1 mm, and over a duration of 30 min.

3 Results and Discussion

3.1 Synthesis

The additives D1—D6 were synthesized according to the synthetic route described in Scheme 1. In the presence of potassium hydroxide, thiadiazole (compound A) reacted with the corresponding chloroacetate via nucleophilic substitution to give thiadiazole derivative B, which later reacted upon the substituted benzaldehyde C via the nucleophilic addition-elimination reactions to obtain the products D1—D5. The hydroxyl group protection of compound E led to the formation of compound F, which subsequently gave compound D6 after the addition-elimination reactions. The reaction mixture was recrystallized in ethanol to give a crude product that was purifed by silica gel column chromatography to give the desired products, which were then characterized by IR and1H NMR spectroscopy.

3.2 Thermal stability

The thermal stability is an important property of oil additives. Figure 1 presents the TGA curves of additives D1-D6. The TGA results indicated that the six prepared additives possessed good thermal stability, and their decomposition temperature was equal to 267, 270, 272, 259, 280, and 293 °C, respectively. However, the decomposition temperature of AMT was 245 °C, indicating that the thermal stability of synthetic additives was higher than that of the starting material (AMT). The reason regarding why all these prepared compounds had their thermal decomposition temperature in excess of 250 °C might be attributed to the presence of an aromatic ring in their structure.

3.3 Copper strip test

Table 3 shows the results of the copper strip corrosion tests using oil sample containing 0.1% of homologues D1—D6 each. After the test, the copper strips exhibited only mild discolouration. The copper corrosion control standard shade guide was used to determine the corrosion level of 1a/1b. According to the results of copper corrosion tests, these synthetic additives demonstrated excellent corrosion inhibiting properties. In addition, we observed that during the same test duration, the corrosion degree did not vary with temperature. However, with the increase in test time, the degree of corrosion was aggravated even when the temperature was kept constant. This fnding means that the test time is generally a major contributor to the corrosion of copper strip by these additives.

Figure 1 TGA curves of additives D1-D6 in nitrogen atmosphere

Table 3 Results of copper strip test under different conditions

3.4 SRV friction and wear tests

3.4.1 Friction-reducing performance

Figure 2 shows the variation of friction coeffcients with time for the 150SN base oil and oil samples containing 0.1% of additives D1—D6 each under a load of 200 N and 400 N, respectively. The oils containing additives D1—D6 each exhibited lower friction coefficients than the 150SN oil, which might be attributed to the formation of boundary lubrication films whose shear strength was considerably less than that of steel matrix on the rubbing surface[2]. The friction coefficient of the oil samples containing additives D3 and D5 apiece which had a hydroxyl group were smaller than that of the oil sample containing D6 without a hydroxyl group in the structure, indicating that the hydroxyl group, the size effect and the space hindrance could affect the frictionreducing performance of oil samples. The existence of hydroxyl group and a smaller space hindrance could stimulate the friction reduction ability. This might be attributed to the formation of hydroxyl-metal complexes and a faster adsorption with the metal surface, resulting in a more stable and faster protective film. The results exhibited in Figure 2 show that the friction coefficient of additives with a hydroxyl group in theortho-position of the phenyl ring decreased significantly as compared to those with the same hydroxyl group in theparaposition. This fnding is probably attributable to the size effect and space hindrance of the latter structure. For example, the additives D4 and D5 with a hydroxyl group in thepara-position are longer in the transverse direction than additives D1 and D3 with a hydroxyl group in theortho-position. The bigger space hindrance can affect the speed of adsorption and reaction of D4—D5 with the metal surface, leading to low effectiveness and less friction-reducing capability as compared to that of D1 and D3. Additives D1, D2 and D3 with similar structure demonstrated the same ranking of friction reduction effciency as evidenced by their alkyl chain length and the content of active sulphur and nitrogen elements despite their difference in the friction-reducing performance. For example, when the additive concentration was 0.1%, the content of sulphur and nitrogen in these samples was 0.1916, 0.1257 (additive D1), 0.1916, 0.1257 (additive D2), and 0.1788, 0.1173 (additive D3), respectively. The first two additives (D1 and D2) had shorter alkyl chain and higher content of sulphur and nitrogen, therefore their friction coefficients were smaller than that of D3. The short-chain derivative is more capable of reacting with the metal surface to generate a flm with low shear strength as compared to the long-chain derivative[32]. The higher levels of sulphur and nitrogen can also accelerate the speed of adsorption and reaction of additives with the metal surface, and both of which might be the factors that could shed light on why the friction coefficients of the samples containing D1 and D2 were lower than those of the samples containing D3 under the same conditions. The difference between D4 and D5 was similar to the above-mentioned phenomenon.

Thus, it can be confirmed that the synthetic compounds containing a hydroxyl group produce superior frictionreducing effect than those without a hydroxyl group, indicating that the existence of hydroxyl group can play an important role in friction-reducing property. In addition, the additives with anortho-hydroxyl groupexhibit better friction-reducing properties than additives withpara-hydroxy groups, indicating that the hydroxyl position can also affect the friction-reducing performance. Among the additives with a similar structure, the length of the alkyl chain and the content of active elements in these additives together can determine the friction-reducing performance. The smaller the size of a substituent, and the higher the content of active elements, the better the friction reduction performance would be.

Figure 2 Variation of friction coefficient with time for base oil containing 0.1% of different additives and neat 150SN oil at 50 °C (under a load of: (a) 200 N, and (b) 400 N, at a stroke of 1 mm, and a frequency of 50 Hz)

3.4.2 The extreme-pressure performance

The extreme-pressure (EP) performance of the base oil and the sample oils containing 0.1% of the additives was also investigated in this study to determine the maximum load under which the oil flm would rupture. The results exhibited in Figure 3 show that the maximum load values of base oil containing synthetic additives were much higher than that of the neat base stock, indicating that the synthetic compounds with a thiadiazole ring used as additives in the 150SN oil had excellent loadcarrying capacity, probably because these compounds could form an S-rich and N-containing complex film with low shear strength under extreme-pressure boundary lubrication conditions. The addition of the six additives improved the extreme-pressure performance of the base oil in different degree. On the whole, the maximum load values of the oil samples containing additives with free hydroxyl groups were greater than that of the additives, the hydroxyl group of which was protected, indicating that the hydroxyl group can improve the extreme-pressure performance. It seems to be that the hydroxyl group can easily react with the metal to form a stable complex, producing a more stable protective film to improve the extreme-pressure performance. Sulphur is one of the most widely used elements in EP lubricant additives, and the hydroxyl groups in these thiadiazole derivatives are active groups, so both are expected to be effective in improving the adsorption capability and chemical reactivity of these derivatives[32]. In addition, additives D1 and D3 with hydroxyl groups in theortho-position could improve the maximum load more effectively as compared to D4 and D5 with hydroxyl groups in thepara-position, which might be attributed to the size effect and space hindrance. By contrast, the additives D4 and D5 with a hydroxyl group in thepara-position are longer in the transverse direction than additives D1 and D3 with a hydroxyl group in theortho-position. This could affect the speed of adsorption and reaction of D4 and D5 with the metal surface, resulting in a lower extremepressure performance. Among the additives with a similar structure, their sulphur content increased in the following order: D2 ≈ D1 ＞ D3, and their load value decreased in the following order: D2 > D1 > D3, indicating that the maximum load values of the three additives were related to their chemical structure and the concentration of sulphur element. It was reported that although the tested lubricants had the same concentration of sulphur element, their extreme-pressure performance might be different because of the difference in the structure of the S-containing additives[33]. This might be caused by the branched alkyl chain in D2, which was more easily adsorbed on the metal surface to form an adsorption flmof lower shear strength than other additives with straight alkyl chain, since the additive with branched chain tends to react with metal to form a lower shear strength flm in comparison with the additive without a branched chain[2]. In summary, it can be concluded that the EP behaviour of the oil samples containing these derivatives had a direct dependence on the hydroxyl group (especially in theortho-position), the sulphur content, and the structure of these additives. In other words, a high concentration of sulphur alone did not mean that it could possess excellent extreme pressure properties. Desirable oil performance could be achieved only when the suitable sulphur content, a well-placed hydroxyl group, and an alkyl group with appropriate chain length were incorporated into the proper molecular structure of the additive.

Figure 3 Load values of 150SN and 150SN containing different additives (at an additive concentration of 0.1%, a stroke of 1 mm, and a frequency of 50 Hz).

3.5 Surface analysis

In order to obtain a direct surface image and elemental composition of the triboflms, the wear scars were imaged using scanning electron microscopy (SEM), and the elemental composition was analysed using EDS technique for possible lubricating mechanism interpretation.

From the SEM morphology of the worn surfaces of steel discs lubricated with the base oil and the base oil containing 0.1% of D1—D6, respectively, it was found out that the worn surface lubricated with the base stock showed obvious signs of scuffing with a wide and deep wear scar, while the worn surfaces lubricated with the base stock containing 0.1% of D1—D6 each seemed to be more smooth with obviously abated scuffng and wear scar. In addition, there were many furrows and grooves on the friction surface lubricated with the base stock, indicating that the adhesive wear occurred. The wear scars of steel discs lubricated by base oil containing 0.1% of D1—D6 each were relatively narrow and shallow, and scuffng was greatly alleviated. This outcome confrming that additives D1—D6 possessed favourable anti-wear properties was consistent with the measured friction coeffcients.

Based on the EDS spectra of the wear scar on the disc surface lubricated by the neat base oil and the base oil containing 0.1% of D1—D6 each, a quantitative main elemental analysis (obtained from EDS spectra) of the tribofilms is given in Table 4. The amount of sulphur deposited on the worn surfaces originated from the lubricant containing 0.1% of each additive was consistently higher than the sulphur content in the tribofilm originated from the base oil. Meanwhile, the worn surface lubricated with base oil containing these additives had higher nitrogen content than that originated from the base oil, indicating that the nitrogencontaining compounds existed on the worn surface lubricated by the base oil containing these additives. The content of nitrogen detected on the worn surface lubricated by base oil containing the additive D6 was extremely low, which could occur probably due to the low activity of D6 because it lacked a hydroxyl group. Nitrogen and sulfur elements are usually present in the adsorption flm or chemically formed flm on the metal surface. The smaller polarity additive D6 which lacks a hydroxyl group is more diffcult to be adsorbed on the metal surface, while only the chemical reaction flm may exist, and in some extremely rare cases the adsorption flm can be formed on the worn surface. Less nitrogencontaining compounds in the tribofilm originated from the additive D6 can lead to less protective film on the worn surface and lower friction-reducing and extreme-pressure performance, which is consistent with the tribological results mentioned above. It is assumed that the additives D1—D6 in 150SN oil reacting on the worn surface involve complicated tribochemical reactions during the friction process and can form a surface protective film composed of sulphates, sulphides and organic nitrogen-containing compounds.

Table 4 EDS elemental analysis of tribofilms formed on worn surfaces (main elements)w, %

4 Conclusions

Six new 1,3,4-thiadiazole Schiff base derivatives (D1-D6) were synthesized, with their structures characterized by1H NMR and IR spectroscopy. Thermal analysis results revealed that the synthetic additives with their decomposition temperature exceeding 250 °C possessed favourable thermal stability. The copper strip test results showed that the additives were not corrosive to copper, revealing satisfactory anti-corrosion capability. Tribological tests indicated that these derivatives were capable of effectively improving the friction-reducing, anti-wear and extreme-pressure performance of the 150SN base oil.

The EDS analysis showed that a protective flm composed of sulphates, sulphides and organic nitrogen-containing compounds was formed on the lubricated metal surface, which was beneficial to the tribological properties of additives contained in the 150SN oil for steel/steel tribopairs contact. These synthetic additives could have potential applications in the engine oil and rolling bearings industries.

Acknowledgements: The authors would like to thank Mr. Liu Jianhui for critically reviewing the manuscript.

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date: 2017-01-24; Accepted date: 2017-04-26.

Professor Chen Hongbo, E-mail: chenhb@dlut.edu.cn.