- Email: [email protected]
Evaluation of antioxidants in lubricating oils by differential thermal analysis and IR spectroscopy
45
Wear, 82 (1982) 45 - 48
EVALUATION OF ANTIOXIDANTS IN LUBRICATING OILS BY DIFFERENTIAL THERMAL ANALYSIS AND IR SPECTROSCOPY
A. K. BISWAS, B. D. CHATTARAJ, Regional Research Laboratory,
I. SURYANARAYANA
Jorhat 785006,
and V. S. B. RAO
Assam (India)
(Received August 14, 1981; in revised form February 10, 1982)
Summary The use of IR spectroscopy and differential thermal analysis in the study of antioxidants and the assessment of lubricant service life is described.
1. Introduction The oxidation stability of lubricating oils is an important criterion for satisfactory performance. High viscosity index lubricating oil base stocks are fortified with several additives to improve performance in service. Air and moisture which are invariably present in the lubrication system cause oxidation of lubricating oils with a resultant increase in the production of sludge, unsaturated components and acidity which cause an increase in the viscosity and ultimately lead to a decline in performance. The stability of lubricating oils towards oxidation is evaluated by static, bomb and dynamic methods. In ASTM 945 [l] (a dynamic test) a special oxidation cell containing 300 ml of oil and an Fe-Cu catalyst is used. The test is conducted at 95 “c with an oxygen flow rate of 3 1 h-l. The change in acidity after 1000 h is recorded and the test is terminated when the acidity is 0.5 - 2.0 mg KOH (g sample))‘. The differential thermal analysis (DTA) of antioxidant and detergent additives used in lubricating oils [ 2, 31 is reported. A quantitative examination of the DTA spectra of antioxidants in a white oil was made. The IR spectra of oxidized white oil without additives and containing 0.5,l.O and 1.5 wt.% additive were also investigated.
2. Experimental details Technical grade white oils of viscosity 9.6 and 12.4 cSt at 37.8 “c were used. The clay used as the base material in the DTA investigations was 0043-1648/82/0000-0000/$02.75
@ Elsevier Sequoia/Printed
in The Netherlands
46
obtained from the International Trading Centre, Calcutta. The antioxidant (LZ 1360) was obtained from Lubrizol India. The viscosity and acid number were measured according to the procedures given in ASTM 445-65 and ASTM D 974 [ 41 respectively. The oxidation test equipment was supplied by Equipments and Instruments Ltd., Calcutta, and consisted of an oil bath together with tube holders, a flowmeter and a stirrer. In the oxidation cell experiment 100 cm3 of white oil containing 0.5 - 1.5 wt.% LZ 1360 and 4.5 g of iron as a catalyst was used. Oxygen was passed through the tubes at a rate of 3 1 h-‘. The bath was maintained at 100 “C using heaters and a stirrer. The experiment was continued for 300 h. Samples were taken every 20 h to monitor changes in the viscosity, the acid number and the carbonyl content by IR spectroscopy using a Perkin-Elmer PE 237 B Infracord instrument. The spectra were recorded at a slow speed in the 1850 - 1600 cm-’ region using a normal slit width. AI1 measurements were made at a temperature of 37 “C. The apparent absorbances were measured using the baseline technique. The samples for IR spectroscopy consisted of 5 1.11of the reaction mixture in 0.5 ml of chloroform. The following procedure was adopted for DTA. Samples were prepared by adding 20 g of white oil (containing 0, 0.5, 1.0 and 1.5 wt.% additive) to 80 g of calcined clay and homogenizing the mixture. The spectra were obtained using 600 mg samples of the mixture and were always taken with a blank. The rate of heating was maintained at 7.5 “C min-‘.
3. Results and discussion It was observed that once oxidation of the white oil commenced carbony1 formation accelerated. As the additive concentration was increased, carbonyl absorption decreased (Table l), indicating oxidation inhibition by the additive. TABLE 1 Inhibition of oxidation of white oil by the additive Time @I
Carbonyl
absorption
for the foJJowing additive
concentrations
0
0.5 wt.%
1.0 wt.%
1.5 wt.%
50 80 100 140 160
2.24 2.28 2.34 3.00 3.78
0.4 0.447 0.52 1.04 1.7
-
-
180 190
4.78 5.24
2.5 2.84
0.42 0.51
0.25 0.35
47
The acid number is affected by the formation of carboxylic acids after prolonged oxidation and increases with increasing carbonyl formation. Because the formation of the carbonyl group is inhibited by the additive, the acid number decreased with increase in the additive concentration, as shown in Table 2. The oil viscosity increased with the time of oxidation, and the rate of change in viscosity with time decreased with additive concentration. The areas under the exothermic and endothermic peaks of the DTA spectra represent the heats of reaction and are influenced by the additive concentration. The area under the exothermic peak at 265 - 300 “C decreased from 3.5 to 0.8 cm2 for an increase in additive concentration from 0 to 1.5 wt.%. The temperatures at which the maximum weight loss was observed and the temperatures of the exothermic and endothermic peaks for four additive concentrations are given in Table 3. The plot of additive concentration versus DTA peak area increases linearly up to 1.0 wt.% additive and then becomes constant, indicating that further increases in additive concentration have no effect on the oxidation. The effectiveness of oxidation inhibitors and their useful service life can be studied by DTA and IR spectroscopy. The methods are quick, simple and reliable, and only a small number of samples are required.
TABLE 2 Stability against oxidation of white oil containing LZ 1360 Additive
Time
(wt.%)
(h)
0
0
Acid number (mg KOH g-l)
-
Viscosity (cSt) 9.6
60 90 120 200
6 9 24 34
14.0 32.6 52.3 68.4
100 120 200 260
9
10.8 18 22
20.2 26.4 38.6 47.0
1.0
100 200 260 300
8 10 12 15
10.8 18.0 24.0 34.3
1.5
100 200 260 300
6 8 10 12
10.3 12.0 17.3 22.9
0.5
at 3 7.8 “C
48 TABLE 3 Differential thermal analysis Additive
Oxidation peak
(wt.%)
Temperature at maximum weight loss rate (“C)
Endothermic peak (“C)
Exothermic peak (“C)
area (cm’)
0 0.5 1.0 1.5
320 290 300 280
355,385 350,360,390 325,425 370,405
330,500,270 265, 290,380,490 270,300,350,515 265, 280,285,380,515
3.5 2.5 0.9 0.8
at at at at
300 295 300 265
“C “C “C “C
Acknowledgment The authors wish to express their gratitude to the Director, Regional Research Laboratory, Jorhat, Assam, for permission to publish the results.
References ASTMStand. D56-D1980, in 1977AnnualBook OfASTMStandards, Part 23, Petroleum Products and Lubricants, ASTM, Philadelphia, PA 19103, 1977. A. Van Commichau, Study of the aging characteristics of oils through DTA, Erdb’l Kohle, 25 (6) (1972) 322. A. Walocha, Nafta (Katowice, Pol.), 6 (1972) 267 - 272. J. J. O’Connor (ed.), Standard Handbook of Lubrication Engineering, McGraw-Hill, New York, pp. 21 - 26.
Wear, 82 (1982) 45 - 48
EVALUATION OF ANTIOXIDANTS IN LUBRICATING OILS BY DIFFERENTIAL THERMAL ANALYSIS AND IR SPECTROSCOPY
A. K. BISWAS, B. D. CHATTARAJ, Regional Research Laboratory,
I. SURYANARAYANA
Jorhat 785006,
and V. S. B. RAO
Assam (India)
(Received August 14, 1981; in revised form February 10, 1982)
Summary The use of IR spectroscopy and differential thermal analysis in the study of antioxidants and the assessment of lubricant service life is described.
1. Introduction The oxidation stability of lubricating oils is an important criterion for satisfactory performance. High viscosity index lubricating oil base stocks are fortified with several additives to improve performance in service. Air and moisture which are invariably present in the lubrication system cause oxidation of lubricating oils with a resultant increase in the production of sludge, unsaturated components and acidity which cause an increase in the viscosity and ultimately lead to a decline in performance. The stability of lubricating oils towards oxidation is evaluated by static, bomb and dynamic methods. In ASTM 945 [l] (a dynamic test) a special oxidation cell containing 300 ml of oil and an Fe-Cu catalyst is used. The test is conducted at 95 “c with an oxygen flow rate of 3 1 h-l. The change in acidity after 1000 h is recorded and the test is terminated when the acidity is 0.5 - 2.0 mg KOH (g sample))‘. The differential thermal analysis (DTA) of antioxidant and detergent additives used in lubricating oils [ 2, 31 is reported. A quantitative examination of the DTA spectra of antioxidants in a white oil was made. The IR spectra of oxidized white oil without additives and containing 0.5,l.O and 1.5 wt.% additive were also investigated.
2. Experimental details Technical grade white oils of viscosity 9.6 and 12.4 cSt at 37.8 “c were used. The clay used as the base material in the DTA investigations was 0043-1648/82/0000-0000/$02.75
@ Elsevier Sequoia/Printed
in The Netherlands
46
obtained from the International Trading Centre, Calcutta. The antioxidant (LZ 1360) was obtained from Lubrizol India. The viscosity and acid number were measured according to the procedures given in ASTM 445-65 and ASTM D 974 [ 41 respectively. The oxidation test equipment was supplied by Equipments and Instruments Ltd., Calcutta, and consisted of an oil bath together with tube holders, a flowmeter and a stirrer. In the oxidation cell experiment 100 cm3 of white oil containing 0.5 - 1.5 wt.% LZ 1360 and 4.5 g of iron as a catalyst was used. Oxygen was passed through the tubes at a rate of 3 1 h-‘. The bath was maintained at 100 “C using heaters and a stirrer. The experiment was continued for 300 h. Samples were taken every 20 h to monitor changes in the viscosity, the acid number and the carbonyl content by IR spectroscopy using a Perkin-Elmer PE 237 B Infracord instrument. The spectra were recorded at a slow speed in the 1850 - 1600 cm-’ region using a normal slit width. AI1 measurements were made at a temperature of 37 “C. The apparent absorbances were measured using the baseline technique. The samples for IR spectroscopy consisted of 5 1.11of the reaction mixture in 0.5 ml of chloroform. The following procedure was adopted for DTA. Samples were prepared by adding 20 g of white oil (containing 0, 0.5, 1.0 and 1.5 wt.% additive) to 80 g of calcined clay and homogenizing the mixture. The spectra were obtained using 600 mg samples of the mixture and were always taken with a blank. The rate of heating was maintained at 7.5 “C min-‘.
3. Results and discussion It was observed that once oxidation of the white oil commenced carbony1 formation accelerated. As the additive concentration was increased, carbonyl absorption decreased (Table l), indicating oxidation inhibition by the additive. TABLE 1 Inhibition of oxidation of white oil by the additive Time @I
Carbonyl
absorption
for the foJJowing additive
concentrations
0
0.5 wt.%
1.0 wt.%
1.5 wt.%
50 80 100 140 160
2.24 2.28 2.34 3.00 3.78
0.4 0.447 0.52 1.04 1.7
-
-
180 190
4.78 5.24
2.5 2.84
0.42 0.51
0.25 0.35
47
The acid number is affected by the formation of carboxylic acids after prolonged oxidation and increases with increasing carbonyl formation. Because the formation of the carbonyl group is inhibited by the additive, the acid number decreased with increase in the additive concentration, as shown in Table 2. The oil viscosity increased with the time of oxidation, and the rate of change in viscosity with time decreased with additive concentration. The areas under the exothermic and endothermic peaks of the DTA spectra represent the heats of reaction and are influenced by the additive concentration. The area under the exothermic peak at 265 - 300 “C decreased from 3.5 to 0.8 cm2 for an increase in additive concentration from 0 to 1.5 wt.%. The temperatures at which the maximum weight loss was observed and the temperatures of the exothermic and endothermic peaks for four additive concentrations are given in Table 3. The plot of additive concentration versus DTA peak area increases linearly up to 1.0 wt.% additive and then becomes constant, indicating that further increases in additive concentration have no effect on the oxidation. The effectiveness of oxidation inhibitors and their useful service life can be studied by DTA and IR spectroscopy. The methods are quick, simple and reliable, and only a small number of samples are required.
TABLE 2 Stability against oxidation of white oil containing LZ 1360 Additive
Time
(wt.%)
(h)
0
0
Acid number (mg KOH g-l)
-
Viscosity (cSt) 9.6
60 90 120 200
6 9 24 34
14.0 32.6 52.3 68.4
100 120 200 260
9
10.8 18 22
20.2 26.4 38.6 47.0
1.0
100 200 260 300
8 10 12 15
10.8 18.0 24.0 34.3
1.5
100 200 260 300
6 8 10 12
10.3 12.0 17.3 22.9
0.5
at 3 7.8 “C
48 TABLE 3 Differential thermal analysis Additive
Oxidation peak
(wt.%)
Temperature at maximum weight loss rate (“C)
Endothermic peak (“C)
Exothermic peak (“C)
area (cm’)
0 0.5 1.0 1.5
320 290 300 280
355,385 350,360,390 325,425 370,405
330,500,270 265, 290,380,490 270,300,350,515 265, 280,285,380,515
3.5 2.5 0.9 0.8
at at at at
300 295 300 265
“C “C “C “C
Acknowledgment The authors wish to express their gratitude to the Director, Regional Research Laboratory, Jorhat, Assam, for permission to publish the results.
References ASTMStand. D56-D1980, in 1977AnnualBook OfASTMStandards, Part 23, Petroleum Products and Lubricants, ASTM, Philadelphia, PA 19103, 1977. A. Van Commichau, Study of the aging characteristics of oils through DTA, Erdb’l Kohle, 25 (6) (1972) 322. A. Walocha, Nafta (Katowice, Pol.), 6 (1972) 267 - 272. J. J. O’Connor (ed.), Standard Handbook of Lubrication Engineering, McGraw-Hill, New York, pp. 21 - 26.