J Therm Anal Calorim (2013) 113:357–363 DOI 10.1007/s10973-013-3090-7
Improvement of the resistance to oxidation of the ecological greases by the additives Jolanta Drabik • Magdalena Trzos
Received: 12 November 2012 / Accepted: 19 February 2013 / Published online: 20 March 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013
Abstract This article presents the results of research aimed at improving the oxidation stability of the ecological greases addressed toward application in machines working in the food industry. In order to improve the functional properties of grease, additives that modify the thermal stability have been added. Then, the influence of the additive on the grease resistance to the oxidation was examined. The results of tests of lubricants containing different types of additives are presented. The thermal examinations of the greases were carried out with the use of scanning differential calorimetry techniques. The lubricated properties of greases were investigated with the use of a four-ball tribotester. Based on the results, the relationships between the kind of additive, the resistance to oxidation and the lubricated properties of the grease were analysed, and the relationships between thermal and antiwear properties of grease were identified. Based on the results of this research, a new formulation of grease was proposed, which meets both the ecological needs and the working conditions in the food industry. Keywords Ecological greases Additives Oxidation stability Tribological properties Lubricant for food industry
Introduction Many studies [1–4] have demonstrated that elevated temperature has a significant impact on the wear process. Owing to J. Drabik M. Trzos (&) Institute for Sustainable Technologies, National Research Institute, ul. Pulaskiego 6/10, 26-600 Radom, Poland e-mail:
[email protected]
widening of the operational temperature of machinery, the high-temperature lubricants (minimum drop temperature of 230 °C) play an important role in many areas of production. Greases are formulated with base oils, thickeners and additives. In order to improve the functional properties of grease, additives that modify the thermal stability have been added to the grease. An important problem is the identification of the relationships between grease structure, composition and its lubricating properties. The investigation of correlation between the nature of the disperse phase and the tribological characteristics of the petroleum oil and the most effective high-temperature greases with respect to the thickener were presented in literature [5]. The resistance to oxidation (expressed in a temperature units) and oxidative stability (expressed in a time units) are properties of great importance for greases working at an elevated temperature. Such properties of lubricants are often identified with the use of standardised methods which take into account the parameters that foster oxidation, such as the temperature, the oxidative factor, and the presence and the kind of the catalyst. Effects on the oxidation resistance are estimated based on the changes of the structural viscosity in comparison to the base oil [6, 7]. Besides the standard methods in research, non-standardised methods of the instrumental analysis are also used. Practical methods include spectral analysis in the infra-red radiation IR and ultraviolet UV, and scanning differential calorimetry (DSC) [8–11]. DSC is a viable alternative to oven ageing, because evaluation is complete in several hours or less. Publications have presented the usefulness of DSC methods, for example, for a thermal analytical characterisation of lubricating greases [12] and for modelling the utilitarian properties of greases [13]. Greases applied in industry have to meet many requirements with both operational and environmental
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358 Fig. 1 Chemical formula of the silica bases of oil
J. Drabik, M. Trzos
CH3 H3 C
CH3
Si
O
CH3
Si
CH3
Si
O
CH3
CH3
dimethyl silicone oil – PMX
CH3
x CH3
CH3
Sio
Me3Sio
SiMe3
Sio
CH3
R=CH2CH2CF3
R
x
fluorine silica oil – FS
y
Table 1 Oil characteristics Property
Viscosity cSt at 100 °C
Test method
ASTM D445
Silica base oil PMX
FS300
FS1000
28
30
74
Viscosity index/VI
ASTM D2270
220
220
Above 220
Flash point/°C
ASTM D 92
240
260
270
Pour point/°C
ASTM D 97
-67
-43
-41
Four-ball wear, scar diameter/mm
ASTM D2266
1.68
0.59
0.63
aspects. Resistance to wear of the friction joins depend, among others, on lubricant quality changes due to ageing [14] and additives that improve the antiwear properties [15]. The research presented in this article concerns lubricants assigned to implementation in the food industry. The tribological contacts in food industry machines are working at temperatures up to 170 °C. Therefore, the lubricants have to meet special ecological properties, and because of working conditions, they have to be resistant to elevated temperatures. Insufficient oxidative stability of grease leads to increased friction and a high risk of damage to the industrial machinery. Therefore, the development of effective lubricating methods, which may contain greases that protect friction elements from direct contact and have good resistance to seizure while working in elevated temperatures, is of great interest. This article presents the results, based on this research, of the developed greases based silica oils. Silica has been well studied and has seen wide application as an agent to reinforce and modify the rheological properties of liquids [16] and improve their tribological properties [2, 17] In the case of silicone oil bases, it is possible to ensure the stable operation of the lubricant at very low temperatures of about -70 °C (silicones) to over 250 °C (fluorinated silicones).
Experimental The methodology encompasses two main phases: the choice of the base oil and the investigation on the additives
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influence on the properties of grease, designed on the basis of selected the oil base. Physicochemical properties of lubricants that were designed on the silica oil basis were estimated in accordance with the standardised test procedure. The oxidative stability, resistance to oxidation and tribological properties were measured. In order to estimate the efficiency of resistance to wear of greases, which have been developed based on different silica oils, namely, a dimethyl silicone and a fluorine silicone, the chosen characteristics of friction and wear were compared. The lubricating properties of the base oil and developed greases and the influence of the modifiers on these properties were assessed using the normalised four-ball test methods based on the ASTM D4172 standard. The standardised wear measurements, the average wear scar diameter (d), the limiting load of the wear (Goz) and the weld point (Pz) were determined. The average wear scar diameter and limiting load of the wear, which characterise the antiwear (AW) properties of the tested oils, were determined after the 3,600-s runs at a constant rotational speed (1450 rpm) and constant load (392 N). The weld point, reflecting the extreme-pressure (EP) properties of the tested oils, was determined after the 10-s runs at a constant rotational speed (1,450 rpm) and a stepwise increasing load (from 78.5 to a maximum of 785 daN) were performed until seizure (welding) of the test balls was observed. Resistance to oxidation was determined based on the oxidation onset temperature (OOT). The results were obtained for samples using the LABSystem SETARAM
Improvement of resistance to oxidation of ecological greases Fig. 2 The X-ray spectrum of the surface of the thickener (a) and a photograph of the scanning (91000) of the thickener AR (b)
(a)
359
(b)
Si
4000 3500
Counts/quality
3000 2500 2000 0
1500 1000 500 0
A1 C 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7
Energy/keV
The comparisons of tribological characteristics for greases using different base oils are presented in Fig. 3. The dimethyl silica oil (PMX) and fluorine silica oils, of different viscosities, namely: FS300 and F1000 (viscosity at 25 °C accordingly: 300 and 1,000 cSt) were compared. The values of variables (Fig. 3a) were calculated as the average values from the three repetitions of the experiment, using standardised method [18]. The friction torque as a (a)
d
1.8 1.6 1.4
500
1
300
0.8
d/mm
1.2
400
0.6
200
0.4
Selection of base oil
100
0.2 0
0 FS300
(b)
FS1000
PMX
16 14
Friction torque/Nm
Greases designed based on the synthetic silica oils (dimethyl silicone—PMX and fluorine silicone—FS) that are characterised by different chemical structures (Fig. 1) were experimentally investigated by comparing their operational properties. The base oils used in the research have different viscosities, and the characteristics of the base oils are presented in Table 1. The silica gel of the layered construction of high thermal resistance was used as a thickener in the silicone lubricants. The thickener X-ray spectrum and structure are presented in Fig. 2. The Roentgen studies have confirmed the high purity of the thickener and the presence of only atoms of the analysed chemical compound in the molecules. The X-ray spectrum of the surface of the thickener confirmed the presence of only typical chemical compounds with silica and oxygen groups.
Goz
700 600
Goz/Nmm–2
TG/DSC apparatuses, which provide information about the temperature at which oxidation begins, while heating in the atmosphere of oxygen. The temperature was extracted from the DSC curve and used for the assessment of the resistance to oxidation of the base oils and compositions. The following parameters were used for DSC: sample 3.5 ± 0.3 mg, the range of the temperature 20–400 °C, the speed of warming 10 °C-1 min, and an atmosphere of O2. The PetroOXYTM test was used as the method of oxidation stability determination. The tests were conducted at 140 °C with a continuous flow of oxygen, and the mass of the sample was determined (10 g). A characteristic parameter of the PetroOXY test is the time when the change of maximum pressure achieves 10 %.
12 10 8 6 4
FS300 FS1000
2
PMX 0 –0 0
5
10
15
20
Time/s
Fig. 3 Lubricating characteristics of greases based on PMX and FS oils a limiting load of the wear (Goz) and average wear scar diameter (d), b friction torque as a function of increasing load
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360
J. Drabik, M. Trzos
function of increasing load (Fig. 3b) were measured using method for research of scuffing [19]. The research results proved that the tribological properties of greases based on fluorine silica oils are better than the greases based on methyl oils. Therefore, in the next step of research, the lubricating compositions based on the fluorine silica oils were designed and investigated. The greases are compounds with the inorganic thickener (AR) and different additives, namely: polymers (A), carbon nanostructures that contain fullerene (B), and graphite (C). The composition and abbreviation of the designed lubricants are given in Table 2.
Table 2 Lubricating compositions Grease abbreviation
Base oil
Additives A/%
B/%
C/%
F
FS1000
–
–
–
F-A3
FS 300
5
–
–
F-A10
FS 1000
5
–
–
F-B10
FS1000
–
0.5
–
F-AB10
FS 1000
4.5
0.5
–
F-C3
FS 300
–
–
1
F-C10
FS 1000
–
–
1
(a)
15 Sample: Additive A, 3,25 mg
10
Heat flow/mW
Fig. 4 The scanning microscopy photos of additives (91000) and DSC curve showing the determination of the OOT on the additives a polymers, b nanostructures of carbon, c graphite
5
0
–5
–10 0
100
200
300
400
500
600
700
800
900
600
700
800
900
Temperature/°C
(b)
4 Sample: Additive B; 3,68 mg
Heat flow/mW
3 2 1 0 –1 –2 0
100
200
300
400
500
Temperature/°C
(c) 18 Sample: Additive C; 3,28 mg
Heat flow/mW
14 10 6 2 –2 –6 0
100
200
300
400
500
600
Temperature/°C
123
700
800
900
361
450
290
400
285
350
280 275 270
1
250
0.8
200
0.6
265 260
100
255
50
0.4 0.2 0
0
250
F 267
F_A3 269
F_A10 280
F_B10 286
F_AB10 285
F_C3 268
F_C10 271
Fig. 5 Influence of the additive type of resistance to oxidation according to DSC 50 45
Oxidation induction time/h
1.2
300
150
OOT
40 35 30 25 20 15 10 5 0
Time /h
F
F_A3
F_A10
F_B10
F_AB10
F_C3
F_C10
21
42
43
25
39
24
44
Fig. 6 Influence of additives on oxidation stability of greases according to PetroOXY
9000 8000 7000 6000
Pz /N
1.4
d
Goz
5000 4000 3000 2000 1000 0 F
F-A10
F-B10
F-C10
F-AB10
Fig. 7 The weld point of silica greases
Results and discussion The additives used in the developed greases differ by structure and resistance to oxidation. Graphical representations and photographs of the additive structures and the results of DSC measurements are presented in Fig. 4. Graphite has been shown to have the best resistance to oxidation. The carbon nanostructures are characterised by
d/mm
295
G oz /Nmm–2
Oxidation stability/°C
Improvement of resistance to oxidation of ecological greases
F
F-A10
F-B10
F-AB10
F-C10
Fig. 8 The limiting load of the wear and wear scar diameters on the balls of silica greases
significantly lower oxidation onset temperatures than both graphite and polymer additives. In the next step of the research, the influence of additives on the oxidative and tribological properties of grease was investigated. Resistance to oxidation and oxidation stability of the investigated greases are presented in Figs. 5 and 6. The research results analysis proved that B additives are the most effective in increasing resistance to oxidation. The additive A improves the resistance to oxidation if the viscosity of the oil is high enough (see Fig. 5 and compare the F-A3 and F-A10 greases). In the case of the C additive, the change of the resistance to oxidation is not significant in comparison with the base grease (F). The carbon nanostructure additive significantly improves the resistance to oxidation of grease but has little impact on the oxidative stability (Fig. 6). Additives A and C have the greatest impacts on improving the oxidative stability of greases. In the case of polymer additive (A), the base oil viscosity is not significant for the oxidative stability of grease. However, base oils with higher viscosities are better for the lubricant composition with the addition of graphite (C), in terms of the oxidising properties. In the next step of research, the oxidative and lubricating properties of greases based on oil of 1000 viscosity were analysed and compared. The grease compositions, particularly additives, influence antiwear properties and can improve the extreme-pressure properties of lubricants. Therefore, both tribological properties were measured and are presented in Figs. 7 and 8. Examples of a wear scar on the ball after friction with the use of different silica greases are presented in Fig. 9. The results of seizure tests on the T-02 machine are summarised in Fig. 7. From this figure, it can be seen that grease F-A10 has the best antiscuffing properties, but the greases F-AB, F-B10, and F-C3 show the same weld points. The wear scares were measured with the use of the optical microscopy Nikon MM-40.
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362
J. Drabik, M. Trzos
Fig. 9 Pictures of wear scar (91000) on ball surface lubricated during friction with silica greases a F, b F-B10, c F-A10, d F-C10, and e F-AB10
Petrooxy/h
DSC/°C
9000
320
40 35
270
30 220
25
170
20 15
120
10 70
5
8000
correlation coefficient R = 0.82
7000
Pz / N
45
Oxidation induction time/h
Oxidation stability/°C
50 370
6000 5000 4000
0
20 F
F_A10
F_B10
F_C10
3000
F_AB10
2000 260
Fig. 10 The oxidative properties of silica greases
265
270
275
280
285
DSC/°C
Pz /N
Goz
Fig. 13 Regression model of relation Pz(DSC) for greases based on fluorine silica oils
450
8000
400
7000
350
6000
300
5000
250
4000
200
3000
150
2000
100
1000
50
0
G oz /Nmm–2
Pz
9000
0 F
F–A10
F–B10
F–C10
F–AB10
Fig. 11 The lubricating properties of silica greases
500
correlation coefficient R = 0.80
Goz /Nmm–2
400
300
200
100
0 265
270
275
280
285
290
DSC/°C
Fig. 12 Regression model of relation Goz(DSC) for greases based on fluorine silica oils
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Analysis of the recorded images of the surface bead, lubricated with silicone grease, showed that abrasive wear is dominant on the ball edge, and local damage was observed. Both additives A and B cause an increase in wear scar diameter in comparison with the base grease. Polymers had an especially negative influence negative on wear properties. Nevertheless, adding both a polymer and carbon nanostructures that contain fullerene can eliminate the increase of wear. The research results have shown the favourable action of graphite and the strongly adverse action of A and B additives with respect to the antiwear properties of the lubricant. However, the addition of polymer can significantly increase the welding point, while graphite can reduce this property. Figures 10 and 11 present the comparison of the influence of additives on oxidative properties and lubricating properties, respectively. The research results proved that additives can significantly improve the oxidative properties of greases; however, they may adversely influence a particular lubricating property. The most interesting result was for greases containing additives of both polymer and the nano-structure of carbon. This grease has much better oxidative properties and lubricating properties, and it is no worse than the grease without additives.
Improvement of resistance to oxidation of ecological greases
The relationships between oxidation and tribological properties were analysed for the all developed greases. The regression models of relation Goz(DSC) and Pz(DSC) as a relationship between resistance to oxidation and antiwear properties of grease are presented in Figs. 12 and 13. The observed decreasing relation Goz(DSC) is caused by graphite additives that have high antiwear properties and low DSC. In the case of the Goz measurement, the temperature is not so high as to exceed the resistance to oxidation, and so the additives play the most important role in the tribological properties. A different situation is observed for weld point. The temperature during friction is high, and so the DSC of grease is of great importance, which is visible in relationship presented in Fig. 13.
Conclusions The research results have shown the good lubricating properties of designed lubricants based on fluorine silica oils. The analysed properties related to both the friction and wear of greases are good enough for application in friction contacts. However, the additives influence the thermal and lubricating properties differently. The graphite additive strongly influences the increase of the antiwear properties of grease. The polymer and the carbon nanostructures that contain fullerene added to grease increase the resistance to oxidation; however, they simultaneously cause a decrease in the antiwear properties compared with grease without additives. Studies have shown the positive action of additives for oxidation resistance, oxidation stability and tribological properties of the tested lubricants. However, their impact is varied, depending on the tested characteristics. Improvement of resistance to the oxidation process can significantly increase the resistance to seizing while decreasing the resistance to wear. The obtained results indicate the possibility of improving some of the properties of greases without decreasing the others by the simultaneous addition of various additives. In the next stage of research, the synergistic effect of appropriately chosen additives that improve the working properties of greases assigned to use in elevated temperatures will be investigated. Acknowledgments This scientific study was executed within the Strategic Project ‘‘Innovative Systems of Technical Support for Sustainable Development of Economy’’ within Innovative Economy Operational Project 2007–2013. This research was sponsored by the Polish Ministry of Science under Grant number NN508 481138.
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