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Vaporization and Oxidation of Poly-α-olefin on Metal Plates
Studies in Surface Science and Catalysis 132 Y. Ivvasawa, N. Oyama and H. Kunieda (Editors) c) 2001 Elsevier Science B.V. All rights reserved.
805
Vaporization and Oxidation of Poly- a -olefin on Metal Plates Kazuaki Hachiya Department of Mechanical Engineering, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan The amounts of a lubricant, poly- a -olefin, vaporized on aluminium, iron, stainless Steel, and copper plates at 150, 170, and 190 °C in the air were measured to a precision of 10"^ g with a balance. The oxidation of the lubricant, caused by the heating at the same temperature as the vaporization experiment, was observed by the absorbance at 1716 cm"^ with a FT-IR spectrophotometer. The changes in the vaporization and oxidation with time were remarkably different between an aluminium plate and a group of iron and stainless steel plates. Taking into account these differences, the three types of the lubricant films (i.e., the films of a fresh lubricant, of a partially oxidized lubricant, and of a polymerized lubricant) appear to exist on the metal plates at 150-190 ° C. 1. INTRODUCTION The seizure occurs between metal surfaces of a machine, which contact with each other. When the temperature on the contact surfaces rises by the seizure, the lubricant is vaporized and oxidized. The conventional experimental methods for the oxidation were to measure the amount of oxygen uptake in lubricant and to analyze the oxidation products of the lubricant [1]. These methods do not seem to be the direct measurements of the seizure occurring on metal surfaces of a machine. In the present experiment, the oil film, whose thickness was the same order as the one of fluid film lubrication, was formed on a metal plate surface with the lubricant of poly- a -olefin (PAO). After the lubricant film was oxidized in the air around the temperature where the seizure occurred on the contact metal surfaces, the vaporization and oxidation of the lubricant were followed by the measurements of the weight and of the absorbance with a FT-IR spectrophotometer, respectively. 2. EXPERIMENTAL The lubricant was poly- a -olefin, LUCANT HC-20 (Mitsui Chemicals, Inc.) with an average molecular weight of 800 and a density of 0.833 gizrv? at 15 Q. Metal plates tested were commercial products of rolled plates, and were aluminium (JIS A105OP, 99.5 % Al), iron (JIS SS400, 99.90 % Fe), stainless steel (JIS SUS430, 82 % Fe and 18 % Cr), and copper (JIS CI 100, 99.90 % Cu). They were polished with alumina of average diameter of 10 - 20 /i m, and were washed with a detergent. After the metal plates were washed with distilled water, they are dried in hot air [2]. The arithmetic average roughnesses, Rg, and the maximum height roughnesses, R^^^, were 0.73 and 12 /x m for an aluminium pate, 0.84 and 7.0 M ni for an iron plate, 0.41 and 10 /z m for a stainless steel plate, and 0.48 and 5.8 /z m for a copper plate, respectively. The plate was 5 cm in length, 6 cm in width, and from 0.1 to 0.5 mm in thickness. After 10 /il of 10 wt% benzene solution of lubricant was spread on a metal plate and the
806 solvent was evaporated, a thin oil film was formed. The lubricant film was heated on a hot plate at 150, 170, or 190 "C in the air. In the present experiment, the heating temperature of a metal plate was adjusted within ±V C, At high temperature, the lubricant was evaporated and oxidized. The amount of lubricant remaining on the metal plate was measured to a precision of 10"^ g with a Sartorius balance BP211D, which was grounded in order to prevent the electrification of the metal plate. The area of the lubricant, spread on a metal plate, was measured by a computer analysis of a picture which was taken with a digital camera. As a result, the film thickness was calculated from the ratio of the lubricant weight to the film area and the density. The change in the infrared absorbance caused by the oxidation of the lubricant was measured with a reflection type of a JASCO spectrophotometer FT/IR-410. After this absorbance was divided by the film thickness, the absorbance per 1 cm could be finally obtained. 3. RESULTS AND DISCUSSION The amounts of PAO vaporized on various metal plates were measured at 150, 170, and 190 ° C. Figure 1 shows the lubricant weight remaining on an aluminium plate as a function of time. The lubricant weight decreased monotonously with time at any temperature. As seen from Figure 1, most of the lubricant was vaporized, but only a small amount of lubricant remained on the plate for a long time at 150, 170, and 190 " C, because several interference fringes, formed by a thin oil film, were observed on the plate even at the end of the experiment. Figure 2 shows the amount of PAO remaining on aluminium, iron, stainless steel, or copper plate at 170 ° C. The weight of the lubricant on a iron plate at 170 *C decreased initially with time, but the vaporization of the lubricant almost stopped at about 20 min. At this time, about 50 % of the lubricant remained on the plate surface. The times, when the vaporization stopped at 150 and 190 ' C, were 100 and 10 min, respectively. The time dependence of the lubricant weight on a stainless steel plate changed similarly to that on the iron plate at 150, 170, or 190° C. On a copper plate, 20 to 30 % of the total weight decreased during 20 min, and then the amount of the remaining lubricant was kept almost constant. When PAO on the metal plate was heated under the same temperature condition as
100 150 time/min Fig. 1. Weights of poly- a -olefin remaining on an alminium plate at 150 (D), 170 ( • ) , and 190 (O) " C as functions of time.
40 60 time / min
80
00
Fig. 2. Weights of poly- a -olefin remaining at 170 " C on aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.
807
40 60 time/min
40 60 80 time / min Fig. 3. Absorbanccap at 170 ° C for aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.
Fig. 4. Area S (A), film thickness d (D), and weight W (O) of poly- a -olefin on an alminium plate at 170 ° C as functions of time.
the above vaporization experiment, an infrared spectrum showed a new peak at 1716 cm''. The appearance of the peak at this wave number would indicate an oxidation of the lubricant (i.e., a stretching vibration of C=0). The apparent absorbance, AbsorbancCap, of the lubricant on the metal plate at 1716 cm'' and 170 *C is shown in Figure 3. After the apparent absorbances for aluminium, iron, and stainless steel plates increased first with time, they decreased a little, and then were kept almost constant. The decrease in the apparent absorbance would be caused by the decrease in the oil film thickness, which was obtained by the ratio of the lubricant weight to the density and the film area, as shown in Figure 4. In order to obtain the absorbance for a constant film thickness, the ordinary absorbance per 1 cm, Absorbance, was calculated from the ratio of the apparent absorbance to the film thickness. Figure 5 shows the time dependence of the Absorbance of the lubricant on an aluminium plate. The Absorbance at any temperature increased with time, and became constant for some time. However, it increased again with time. The Absorbances for aluminium, iron, stainless steel, and copper plates at 170 *C are shown in Figure 6. The
lU
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8
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6
c cs
1
1
T —
I
/
1
J
Jd
^ 4 < 2 jfljarir? • • ^ - ^ — • "T *'
50
100 150 200 time / min
250
Fig. 5. Absorbances of poly-a-olefin on an alminium plate at 150 (D), 170 ( • ) , and 190 (0)°C as functions of time.
0
20
40
1
60 80 time / min
—1
100 120
Fig. 6. Absorbances of poly- a -olefin at 170 ° C on aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.
Absorbances of the lubricant on iron and stainless steal plates at 170 * C increased with time, but became almost constant after about 20 min, respectively. The time, when a constant Absorbance was reached, became short with increasing temperature. The Absorbance for a copper plate increased monotonously with time, and was relatively smaller than the Absorbances for iron, stainless steal, and aluminium plates. The Absorbance of the lubricant at 1716 cm"^ increased with the progress of the oxidation in Figures 5 and 6. The assignment of this peak was done by the following way. When stearic acid was spread on a fresh surface of an aluminium plate at room temperature, the infrared spectrum showed the peaks at 1698 (a stretching vibration of C=0) and 940 (a bending vibration of 0-H) cm"^ However, when the same acid was spread on an aluminium surface that was formed by heating at 190 ° C for 5 min and then was cooled to room temperature, the peak at 940 cm'^ disappeared, but the one at 1698 cm'^ remained. This would indicate that an oxide film was formed on a fresh metal surface, when a metal plate was heated at 190 °C for more than 5 min. Furthermore, the spectrum of the stearic acid, observed on the oxidized aluminium surface, was similar to the spectrum of the oxidized PAO on the metal plate that was heated at 150, 170, or 190 ° C. This would mean that the oxidized PAO is a carboxylic acid and reacts with the oxidized metal surface [2]. Because such a thin oil film is formed on the oxidized aluminium surface, a small amount of lubricant would remain on the plate for a long time as shown in Figure 1. Bowden and Tabor classify the metal plates into two types: (a) in which chemical attack is absent or slight (iron, aluminium, nickel, and chromium), and (b) in which chemical attack is marked (copper) [3]. Then, the difference in the Absorbance between the copper and the other metals in Figure 6 is considered to arise from the difference in the reactivity between the oxidized PAO and the metal plates. The amounts of the remaining lubricant for iron and stainless steel plates are different from that for an aluminium plate in Figure 2. This would indicate that a different substance with a higher boiling point (i.e., a larger molecular weight) than PAO is newly produced on the iron or stainless steel plate. In other words, there seem to exist at least three types of the lubricant films on the metal plates: the one is the lubricant film which is not oxidized, and the other are two types of oxidized films of the lubricants, whose molecules are partially oxidized and are polymerized to form larger molecules (i.e., sludge [4]), respectively. On an aluminium plate, the polymerized molecule would be hardly produced in comparison with the partially oxidized one. Although the partially oxidized molecule reacts strongly with the oxidized aluminium plate, the PAO, not oxidized, vaporizes from the plate. Since the concentration of the partially oxidized lubricant increases on the metal plate because of the PAO vaporization, the absorbance for aluminium would increase again as shown in Figure 5. Because the increase in the molecular weight causes the increase in viscosity or in coefficient of friction, the formation of sludge might be one reason why the seizure occurs between two metal surfaces of a machine. REFERENCES 1. E.L. Lederer, Petroleum, 31 (1935) 44. 2. J.V. Sanders and D. Tabor, Proc. Roy. Soc. A, 204 (1951) 525. 3. F.P. Bowden and D. Tabor, The Friction and Lubricant of Solids, Clarendon Press, Oxford, 1954, pp. 200-227. 4. M. Rasberger, Chemistry and Technology of Lubricants (R. M. Mortier and S. T. Orszulik, ed.) , Blackie A & P, London, 1997, pp. 104-106.
805
Vaporization and Oxidation of Poly- a -olefin on Metal Plates Kazuaki Hachiya Department of Mechanical Engineering, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan The amounts of a lubricant, poly- a -olefin, vaporized on aluminium, iron, stainless Steel, and copper plates at 150, 170, and 190 °C in the air were measured to a precision of 10"^ g with a balance. The oxidation of the lubricant, caused by the heating at the same temperature as the vaporization experiment, was observed by the absorbance at 1716 cm"^ with a FT-IR spectrophotometer. The changes in the vaporization and oxidation with time were remarkably different between an aluminium plate and a group of iron and stainless steel plates. Taking into account these differences, the three types of the lubricant films (i.e., the films of a fresh lubricant, of a partially oxidized lubricant, and of a polymerized lubricant) appear to exist on the metal plates at 150-190 ° C. 1. INTRODUCTION The seizure occurs between metal surfaces of a machine, which contact with each other. When the temperature on the contact surfaces rises by the seizure, the lubricant is vaporized and oxidized. The conventional experimental methods for the oxidation were to measure the amount of oxygen uptake in lubricant and to analyze the oxidation products of the lubricant [1]. These methods do not seem to be the direct measurements of the seizure occurring on metal surfaces of a machine. In the present experiment, the oil film, whose thickness was the same order as the one of fluid film lubrication, was formed on a metal plate surface with the lubricant of poly- a -olefin (PAO). After the lubricant film was oxidized in the air around the temperature where the seizure occurred on the contact metal surfaces, the vaporization and oxidation of the lubricant were followed by the measurements of the weight and of the absorbance with a FT-IR spectrophotometer, respectively. 2. EXPERIMENTAL The lubricant was poly- a -olefin, LUCANT HC-20 (Mitsui Chemicals, Inc.) with an average molecular weight of 800 and a density of 0.833 gizrv? at 15 Q. Metal plates tested were commercial products of rolled plates, and were aluminium (JIS A105OP, 99.5 % Al), iron (JIS SS400, 99.90 % Fe), stainless steel (JIS SUS430, 82 % Fe and 18 % Cr), and copper (JIS CI 100, 99.90 % Cu). They were polished with alumina of average diameter of 10 - 20 /i m, and were washed with a detergent. After the metal plates were washed with distilled water, they are dried in hot air [2]. The arithmetic average roughnesses, Rg, and the maximum height roughnesses, R^^^, were 0.73 and 12 /x m for an aluminium pate, 0.84 and 7.0 M ni for an iron plate, 0.41 and 10 /z m for a stainless steel plate, and 0.48 and 5.8 /z m for a copper plate, respectively. The plate was 5 cm in length, 6 cm in width, and from 0.1 to 0.5 mm in thickness. After 10 /il of 10 wt% benzene solution of lubricant was spread on a metal plate and the
806 solvent was evaporated, a thin oil film was formed. The lubricant film was heated on a hot plate at 150, 170, or 190 "C in the air. In the present experiment, the heating temperature of a metal plate was adjusted within ±V C, At high temperature, the lubricant was evaporated and oxidized. The amount of lubricant remaining on the metal plate was measured to a precision of 10"^ g with a Sartorius balance BP211D, which was grounded in order to prevent the electrification of the metal plate. The area of the lubricant, spread on a metal plate, was measured by a computer analysis of a picture which was taken with a digital camera. As a result, the film thickness was calculated from the ratio of the lubricant weight to the film area and the density. The change in the infrared absorbance caused by the oxidation of the lubricant was measured with a reflection type of a JASCO spectrophotometer FT/IR-410. After this absorbance was divided by the film thickness, the absorbance per 1 cm could be finally obtained. 3. RESULTS AND DISCUSSION The amounts of PAO vaporized on various metal plates were measured at 150, 170, and 190 ° C. Figure 1 shows the lubricant weight remaining on an aluminium plate as a function of time. The lubricant weight decreased monotonously with time at any temperature. As seen from Figure 1, most of the lubricant was vaporized, but only a small amount of lubricant remained on the plate for a long time at 150, 170, and 190 " C, because several interference fringes, formed by a thin oil film, were observed on the plate even at the end of the experiment. Figure 2 shows the amount of PAO remaining on aluminium, iron, stainless steel, or copper plate at 170 ° C. The weight of the lubricant on a iron plate at 170 *C decreased initially with time, but the vaporization of the lubricant almost stopped at about 20 min. At this time, about 50 % of the lubricant remained on the plate surface. The times, when the vaporization stopped at 150 and 190 ' C, were 100 and 10 min, respectively. The time dependence of the lubricant weight on a stainless steel plate changed similarly to that on the iron plate at 150, 170, or 190° C. On a copper plate, 20 to 30 % of the total weight decreased during 20 min, and then the amount of the remaining lubricant was kept almost constant. When PAO on the metal plate was heated under the same temperature condition as
100 150 time/min Fig. 1. Weights of poly- a -olefin remaining on an alminium plate at 150 (D), 170 ( • ) , and 190 (O) " C as functions of time.
40 60 time / min
80
00
Fig. 2. Weights of poly- a -olefin remaining at 170 " C on aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.
807
40 60 time/min
40 60 80 time / min Fig. 3. Absorbanccap at 170 ° C for aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.
Fig. 4. Area S (A), film thickness d (D), and weight W (O) of poly- a -olefin on an alminium plate at 170 ° C as functions of time.
the above vaporization experiment, an infrared spectrum showed a new peak at 1716 cm''. The appearance of the peak at this wave number would indicate an oxidation of the lubricant (i.e., a stretching vibration of C=0). The apparent absorbance, AbsorbancCap, of the lubricant on the metal plate at 1716 cm'' and 170 *C is shown in Figure 3. After the apparent absorbances for aluminium, iron, and stainless steel plates increased first with time, they decreased a little, and then were kept almost constant. The decrease in the apparent absorbance would be caused by the decrease in the oil film thickness, which was obtained by the ratio of the lubricant weight to the density and the film area, as shown in Figure 4. In order to obtain the absorbance for a constant film thickness, the ordinary absorbance per 1 cm, Absorbance, was calculated from the ratio of the apparent absorbance to the film thickness. Figure 5 shows the time dependence of the Absorbance of the lubricant on an aluminium plate. The Absorbance at any temperature increased with time, and became constant for some time. However, it increased again with time. The Absorbances for aluminium, iron, stainless steel, and copper plates at 170 *C are shown in Figure 6. The
lU
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8
~a5
6
c cs
1
1
T —
I
/
1
J
Jd
^ 4 < 2 jfljarir? • • ^ - ^ — • "T *'
50
100 150 200 time / min
250
Fig. 5. Absorbances of poly-a-olefin on an alminium plate at 150 (D), 170 ( • ) , and 190 (0)°C as functions of time.
0
20
40
1
60 80 time / min
—1
100 120
Fig. 6. Absorbances of poly- a -olefin at 170 ° C on aluminium (O), iron (D), stainless steel ( • ) , and copper ( • ) plates as functions of time.
Absorbances of the lubricant on iron and stainless steal plates at 170 * C increased with time, but became almost constant after about 20 min, respectively. The time, when a constant Absorbance was reached, became short with increasing temperature. The Absorbance for a copper plate increased monotonously with time, and was relatively smaller than the Absorbances for iron, stainless steal, and aluminium plates. The Absorbance of the lubricant at 1716 cm"^ increased with the progress of the oxidation in Figures 5 and 6. The assignment of this peak was done by the following way. When stearic acid was spread on a fresh surface of an aluminium plate at room temperature, the infrared spectrum showed the peaks at 1698 (a stretching vibration of C=0) and 940 (a bending vibration of 0-H) cm"^ However, when the same acid was spread on an aluminium surface that was formed by heating at 190 ° C for 5 min and then was cooled to room temperature, the peak at 940 cm'^ disappeared, but the one at 1698 cm'^ remained. This would indicate that an oxide film was formed on a fresh metal surface, when a metal plate was heated at 190 °C for more than 5 min. Furthermore, the spectrum of the stearic acid, observed on the oxidized aluminium surface, was similar to the spectrum of the oxidized PAO on the metal plate that was heated at 150, 170, or 190 ° C. This would mean that the oxidized PAO is a carboxylic acid and reacts with the oxidized metal surface [2]. Because such a thin oil film is formed on the oxidized aluminium surface, a small amount of lubricant would remain on the plate for a long time as shown in Figure 1. Bowden and Tabor classify the metal plates into two types: (a) in which chemical attack is absent or slight (iron, aluminium, nickel, and chromium), and (b) in which chemical attack is marked (copper) [3]. Then, the difference in the Absorbance between the copper and the other metals in Figure 6 is considered to arise from the difference in the reactivity between the oxidized PAO and the metal plates. The amounts of the remaining lubricant for iron and stainless steel plates are different from that for an aluminium plate in Figure 2. This would indicate that a different substance with a higher boiling point (i.e., a larger molecular weight) than PAO is newly produced on the iron or stainless steel plate. In other words, there seem to exist at least three types of the lubricant films on the metal plates: the one is the lubricant film which is not oxidized, and the other are two types of oxidized films of the lubricants, whose molecules are partially oxidized and are polymerized to form larger molecules (i.e., sludge [4]), respectively. On an aluminium plate, the polymerized molecule would be hardly produced in comparison with the partially oxidized one. Although the partially oxidized molecule reacts strongly with the oxidized aluminium plate, the PAO, not oxidized, vaporizes from the plate. Since the concentration of the partially oxidized lubricant increases on the metal plate because of the PAO vaporization, the absorbance for aluminium would increase again as shown in Figure 5. Because the increase in the molecular weight causes the increase in viscosity or in coefficient of friction, the formation of sludge might be one reason why the seizure occurs between two metal surfaces of a machine. REFERENCES 1. E.L. Lederer, Petroleum, 31 (1935) 44. 2. J.V. Sanders and D. Tabor, Proc. Roy. Soc. A, 204 (1951) 525. 3. F.P. Bowden and D. Tabor, The Friction and Lubricant of Solids, Clarendon Press, Oxford, 1954, pp. 200-227. 4. M. Rasberger, Chemistry and Technology of Lubricants (R. M. Mortier and S. T. Orszulik, ed.) , Blackie A & P, London, 1997, pp. 104-106.