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Thermo-rheological and tribological properties of novel bio-lubricating greases thickened with epoxidized lignocellulosic materials
G Model JIEC 4746 No. of Pages 7
Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
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Thermo-rheological and tribological properties of novel bio-lubricating greases thickened with epoxidized lignocellulosic materials E. Cortés-Triviño* , C. Valencia, M.A. Delgado, J.M. Franco Pro2TecS — Chemical Process and Product Technology Research Centre, Department of Chemical Engineering, University of Huelva, 21071 Huelva, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 November 2018 Received in revised form 29 April 2019 Accepted 21 August 2019 Available online xxx
We examined the rheological and tribological behavior of novel formulations based on castor oil and epoxidized cellulose pulp intended for use as biodegradable lubricating greases. Epoxidized cellulose pulp was found to thicken castor oil to a variable extent depending on its modification degree and the epoxide compound. Greases were subjected to small-amplitude oscillatory shear tests, evaluating the temperature-dependence of the plateau modulus. In addition, friction coefficient and wear were determined in a steel–steel ball-on-three-plates tribological configuration, at two different temperatures (25 and 95 C), generally obtaining smaller values of both parameters when using aromatic diepoxides instead of aliphatic to modify the cellulose pulp. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Bio-lubricating greases Epoxidized cellulose Frictional behavior Rheology
Introduction Biobased lubricating greases have aroused especial attention in recent years owing to their non-polluting effects on the environment [1]. Lubricating greases are semi-solid gel-like systems usually consisting of a mineral or synthetic base oil, a thickener (usually a metal soap) and an additive package that is intended to improve specific beneficial properties [2]. Most of these components are petroleum-derived products and hence obtained from finite resources; also, they can have detrimental effects on human health and ecosystems. These drawbacks have raised growing concern about the potential environmental damage caused by lubricating greases and led the scientific community to explore effective ways to replace some of their components with naturally occurring alternatives in order to develop more environmentally friendly formulations. Although the most feasible choice in this respect is replacing mineral and synthetic oils with vegetable oils [3,4], there have also been attempts at using biobased thickeners (e.g., chemically modified biopolymers [5,6]) and biobased additives (e.g., natural antioxidants or anti-wear agents [7]). The hydroxyl and epoxy groups or polyunsaturation in some vegetable oils provide special chemical functionalities that enhance lubricating properties [2]. Castor oil has been extensively investigated in this respect on the grounds of its wide availability,
* Corresponding author at: Departamento de Ingeniería Química, Campus de “El Carmen”, Universidad de Huelva, 21071 Huelva, Spain. E-mail address: [email protected] (E. Cortés-Triviño).
favorable temperature-dependence of its viscosity and optimum lubrication properties at high temperatures relative to most vegetable oils [8]. However, finding appropriate bio-thickeners for these oil media remains a major challenge since some of the required functional properties (e.g., gel-like behavior, thermal resistance, lubrication efficiency) are still unsuitable for use in completely bio-based formulations intended to replace conventional lubricating greases [9]. Isocyanate-functionalized cellulose pulp dispersions in castor oil have proved excellent candidates for this purpose [5] as they effectively mimic the rheological and tribological behavior of conventional lithium soap-based lubricating greases. Although the resulting polyurethane gel-like materials are assumed to be inert, diisocyanates are generally toxic and hazardous, which precludes their use as green ingredients in completely renewable formulations and sustainable industrial processes. This shortcoming has been addressed by chemically modifying cellulose pulp through epoxidation [10]. Epoxidized cellulose pulp gel-like dispersions in castor oil possess appropriate rheological properties and good physical stability. However, some of their functional properties (especially their tribological and rheological characteristics), and their temperature-dependence, require further investigations before these biobased formulations can be deemed suitable lubricants. The primary purpose of these materials is lubricating two moving surfaces in contact in order to reduce the resulting friction and wear. This requires the gel-like colloidal system formed by the thickener and base oil to withstand deformation when a shear stress is applied so that elastic and viscous forces can be balanced in order to avoid grease leakage and facilitate the release of base oil
https://doi.org/10.1016/j.jiec.2019.08.052 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: E. Cortés-Triviño, et al., Thermo-rheological and tribological properties of novel bio-lubricating greases thickened with epoxidized lignocellulosic materials, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.052
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into the tribological contact area. In fact, the base oil and thickener constitute a complex rheological system [11] whose characteristics dictate whether an effective lubricant film will be formed. As a result, the efficiency of a lubricant in reducing friction is governed mainly by the viscosity of the oil and the operating conditions, whose mutual relationship is often expressed in the form of Stribeck curves. The presence of polymeric components or classical thickener particles is known to increase the thickness and viscosity of the fluid film, and hence to decrease friction coefficients and wear as compared to the base oil alone [12]. Some operational variables such as shear stress, temperature and normal load, among others, can alter the microstructural network of a lubricating grease and the viscosity of the bleeding oil, thereby decisively affecting their tribological behavior [13]. Based on the foregoing, and on the results of preliminary research into the topic [10], the main goal of this work was to assess the suitability of epoxidized cellulose pulp-based oleogels as efficient lubricating greases by examining their rheological and tribological responses as a function of temperature and the nature of the epoxidized cellulose pulp used as biogenic thickening agent.
1 mm gap). The tests were conducted in a frequency range of 0.03– 100 rad/s within the linear viscoelastic region (LVR), at selected temperatures over the range 25–150 C. Tribological characterization
Cellulose pulp (CP) from Eucalyptus globulus was kindly supplied by ENCE, S.A. (Huelva, Spain). Neopentyl glycol diglycidyl ether (NPGDGE), bisphenol A diglycidyl ether (BADGE), resorcinol diglycidyl ether (RDGE) and trimethylolpropane triglycidyl ether (TMPTGE), all from Sigma-Aldrich, were used as epoxide modifiers. Castor oil was supplied by Guinama (Valencia, Spain).
The oleogel samples were characterized for friction in a ball-onthree plates tribological cell coupled to the Physica MCR 501 rheometer. The ensemble comprised a lower measuring geometry with three 45 inclined steel plates (1.4301 AISI 304, 0.21 mm roughness, 80 HRB hardness) where the lubricant was placed and a rotating shaft holding a 6.35 mm diameter steel bearing ball (1.4401 grade 100 AISI 316). Details of the cell design, and the procedure used to calculate normal forces and sliding velocities for this geometry, can be found elsewhere [14]. Pure sliding condition was selected as the most adverse conditions to evaluate both friction and wear. The friction coefficient was monitored at 25 and 95 C by applying a rotational speed sweep from 0.01 to 1000 rpm at a normal load of 20 N. The stationary friction coefficient was determined at the same temperatures by setting a normal force of 20 N and a constant rotational speed of 10 rpm, within the mixed friction regime, and till reaching a steady state value (for 10 min). Each test was replicated 5 times, using fresh lubricating grease in each run. The topography of the wear scar surface left in the steel plates by the constant rotational speed tests was examined by scanning electron microscopy (SEM) on a JXA-8200 SuperProbe (JEOL) microscope. Samples were also subjected to electron probe microanalysis (EPMA) at 15 keV in order to determine the elementary composition of the worn surfaces by energy-dispersive spectroscopy (EDS). All measurements were made in triplicate.
Preparation of epoxidized cellulose pulp-based oleogels
Results and discussion
The starting cellulose pulp was modified with various di-or triepoxides as described elsewhere [10]. Table 1 shows the input CP/ epoxide compound weight ratios used to obtain the differently epoxidized CP samples (ECPs) for thickening the castor oil, as well as the resulting epoxidation degree (determined according to ISO 3001:1999(E)). The ECP samples were subsequently dispersed in castor oil [10], by using a 5% (w/w) concentration in NPGDGEbased samples and a 4% (w/w) in those epoxidized with the other epoxides. (Table 1). The chemical interaction between residual epoxy groups in the ECPs and hydroxyl groups in castor oil led to physically stable oleogels similar to lubricating greases in appearance and consistency.
Thermorheological response
Experimental section Materials
Rheological measurements Small-amplitude oscillatory shear (SAOS) tests were performed on a Physica MCR 501 controlled-stress rheometer from Anton Paar GmbH (Graz, Austria) equipped with a Peltier temperature control system using a serrated plate-plate geometry (25 mm diameter,
All epoxidized cellulose pulp dispersions in castor oil exhibited a typical gel-like behavior closely resembling the rheological response of conventional lubricating greases [15,16]; thus, their storage modulus (G0 ) exceeded their loss modulus (G00 ) throughout the frequency and temperature ranges examined. By way of example, Fig. 1 illustrates the frequency-dependence of the storage and loss moduli for a selected epoxidized cellulose pulp-based oleogel (OECPN-5) over the temperature range 25–150 C. As can be seen, the mechanical spectra exhibited a minimum in G00 and a power-law dependence of G0 on the frequency, which is typical of highly structured colloidal particle gels [17]. Moreover, raising the temperature to 150 C decreased the storage and loss moduli of the oleogels but only slightly. This result contrasts with the typical thermorheological behavior of conventional metal soap-thickened greases, whose SAOS functions decrease dramatically above a critical temperature (e.g., ca. 110 C for lithium greases) as a consequence of structural changes in the thickener network [18].
Table 1 Preparation conditions used to obtain the biogreases. Oleogel code OECPN-1 OECPN-2 OECPN-5 OECPN-10 OECPN-20 OECPB OECPR OECPT OECPN
Epoxy compound NPGDGE
BADGE RDGE TMPTGE NPGDGE
CP/epoxy wt. ratio
Epoxy index (mol/kg)
Thickener wt%
1/1 1/2 1/5 1/10 1/20 1/2 1/2 1/5 1/5
0.6 1.6 2.2 3.6 3.8 2.2 2.2 2.2 2.2
5 5 5 5 5 4 4 4 4
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greases obtained with an aromatic epoxidized cellulose pulpbased thickener [10]. By contrast, the epoxy index seemingly had no appreciable influence on Ea. Frictional behavior
Fig. 1. Variation of the storage modulus (G0 , closed symbols) and loss modulus (G00 , open symbols) for sample OECPN-5 as function of temperature.
For easier understanding of the temperature dependence, Fig. 2 shows the variation of the plateau modulus (GN0, defined elsewhere [19]) with the reciprocal temperature for all epoxidized cellulose pulp-based lubricating greases studied. As can be seen, the variation conformed to the following Arrhenius-type equation: Ea
G0N ¼ AeRT
ð1Þ
where A is the pre-exponential factor, Ea the activation energy, R the gas constant (8.314 J/mol K) and T the absolute temperature. As can be seen, the activation energy, obtained by fitting the curve for each oleogel to Eq. (1) was noticeably low (1.5–5.3 kJ/mol) throughout the temperature range, which suggests a weak temperature-dependence of all samples resulting viscoelastic moduli barely affected by the temperature. Overall, the Ea values are similar to those for commercial lubricating greases in the low temperature range (i.e., below the critical temperature). However, they are much lower than the values typically found above the critical temperature, which is usually dependent on the type of metal soap used as thickener [20]. Although the viscoelastic functions were only slightly influenced by temperature in all samples, using aromatic difunctional epoxides to modify the cellulose pulp led to an increased thermal susceptibility as reflected in also increased activation energy values (Ea = 5.1– 5.3 kJ/mol). This small increase in activation energy may have resulted from increased microstructural interconnection in the
Fig. 3 plots the evolution of the friction coefficient with rotational speed for sample OECPN-5 as obtained by using the tribocell with a 20 N normal load at 25 and 95 C. The curves for the oleogel are compared with the frictional response of the base oil. As can be seen, the epoxidized thickener has a substantial impact on frictional behavior, which was also dependent on temperature and the sliding velocity [21]. Adding ECP to castor oil reduced the friction coefficient in the thickener-dominated region (i.e., under the boundary and mixed lubrication regimes), but mainly at low temperatures. This behavior in the low-speed region may be ascribed to the influence of residual epoxidized cellulose fibers, which may be deposited onto the contact area at low rotational speeds, reducing the asperity contact [22,23]. However, the friction coefficient for the biogreases at the beginning of the full fluid film lubrication regime seems to be slightly higher than that for castor oil as a result of the much higher bulk viscosity. Consistent with previous results for metal soap- and polymerbased lubricating greases [22], the friction curves at 95 C for all lubricants, including castor oil, were largely bell-shaped between the boundary and mixed lubrication regions. At very low sliding velocities, the oleogels had much lower friction coefficients than the base oil, so the friction curves approached one another and eventually converged on a near-identical maximum value as the sliding velocity was increased. At 25 C, however, the friction coefficient levelled off at low sliding velocities and then decreased to a minimum value in the mixed lubrication region. Thus, the curves obtained at this temperature exhibit the typical evolution of the Stribeck curves, comprising the three characteristic lubrication regions (boundary, mixed and hydrodynamic), whereas those obtained at higher temperatures only exhibited the boundary region. Because all tribological tests were done under identical conditions as regards surface geometry, roughness and material combination, the friction coefficient may be plotted against Hersey’s number to obtain typical Stribeck curves. This parameter is used to normalize the abscissa of a Stribeck curve [24] in order to consider the combined influence of lubricant viscosity (h), rotational speed (V) and normal force (FN) on journal bearings. For the configuration used here and other similar geometries [24], the following modification of the dimensionless Hersey number, i.e. the so-called “hydrodynamic Stribeck parameter”, must be
Fig. 2. Variation of the plateau modulus (symbols) with temperature and fitting to the Arrhenius equation (lines) with their corresponding activation energy values.
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Fig. 3. Variation of the friction coefficient with the sliding velocity for a selected biogrease (OECPN-5) and the base (castor) oil upon application of a 20 N normal load at 25 and 95 C.
defined: S¼
h us FN
ð2Þ
having dimensions of [L1], where us is the sliding velocity. At a constant normal load and velocity, the biolubricant viscosity seems to strongly influence the tribological behavior of greases. Thus, a high viscosity favors hydrodynamic lubrication, whereas a very low viscosity results in friction coefficients in the boundary lubrication region [25]. Although the previous relation was developed for Newtonian lubricating oils, it is also useful to obtain information about the tribological behavior of lubricating greases [26]. This requires expressing the viscosity in terms of the base oil and thickener. However, as found elsewhere [27], subjecting grease to extreme shearing for a long time (e.g., in tribological tests) results in considerable microstructural degradation that leads to the separation between oil and thickener, i.e., the “oil bleeding” effect. Therefore, with non-Newtonian systems, the viscosity of the base oil can be, in principle, directly introduced in the expression for the hydrodynamic Stribeck parameter [26], as follows, S¼
hBO us FN
ð3Þ
where hBO is the Newtonian viscosity of the base oil. Parameter S allowed us to directly compare the friction coefficients for epoxidized cellulose pulp-based biogreases at 25 and 95 C simply by substituting the viscosity of castor oil at those temperatures. No
piezoviscosity coefficient (α) the equation since the base oil used was always the same [28]. Fig. 4 shows the master Stribeck curves obtained by overlapping the friction coefficients for epoxidized cellulose pulp-based biogreases as a function of the epoxy index, chemical structure of the epoxy compound and temperature. Most frictional curves obtained at both temperatures roughly overlapped in the manner of Stribeck curves, thus exhibiting the typical lubrication regions. However, this overlapping was not clearly observed in the hydrodynamic region, i.e. at high sliding velocities, where the bulk grease viscosity, and hence the type of thickener used, seem to have a decisive influence on the frictional behavior of the greases. Also, there were some differences in overlap in the mixed lubrication region at 25 C, especially between cellulose pulp samples epoxidized with different compounds (Fig. 4b). Such differences indicate that using the viscosity of the base oil, hBO, in the expression for the Stribeck parameter (Eq. (2)) does not allow the frictional behavior of lubricating greases to be accurately described. As shown below, the rate of grease flow into the ball-ondisc contact area increased in the mixed lubrication region, where entrainment of epoxidized cellulose particles could considerably decreased the friction coefficient (especially at 25 C). To what extent the friction coefficient is reduced in the mixed lubrication region depends on the particular thickener (specifically, on the epoxy index and chemical structure). At 95 C, however, the viscosity of castor oil prevailed in all formulations (due to the oil bleeding effect) and all curves overlapped irrespective of the particular thickener, probably as a result of temperature-induced softening of the oleogels reducing the differences in friction coefficient among the biogreases. It should be noted that the friction coefficients for the biogreases exceeded that for the base oil with full fluid film lubrication and thus depended on the bulk rheological behavior of the biogreases [22,23]. Therefore, the mechanism behind full fluid film formation appears markedly influenced by both the epoxy index and the chemical structure of the thickener. Epoxidation of lignocellulosic materials was previously found to increase their compatibility with castor oil [29]. Thus, the higher the epoxy index was, the better was solvency between the epoxidized cellulose pulp and castor oil. However, continuous over-rolling during tribological tests results in substantial shearinduced structural breakdown and facilitates release of the base oil, thereby favoring formation of a fluid film [27]. Nevertheless, the biogrease OECPN-20, with a high epoxy index, developed an extended crosslinked network that may hinder the release of oil at the lower sliding speeds. Once a steady lubricant film has formed, the friction coefficient seems to be considerably decreased as a
Fig. 4. Plot of the Stribeck parameter against the friction coefficients for the biogreases.
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consequence of the low viscosity and more effective replenishment of bulk grease in the contact area, which may have extended the mixed lubrication region to higher sliding speeds (Fig. 4a). Consequently, improved compatibility between ECPs and castor oil may delay formation of the full fluid film until high sliding velocities are reached, thus shifting the transition point from the mixed lubrication region to the hydrodynamic region to the right [26]. The influence of the nature of the epoxy compound was assessed by examining the friction curves obtained by using different oleogels with an identical epoxy index as lubricating greases (Fig. 4b). As noted earlier, cellulose pulp epoxidized with an aliphatic compound forms oleogels with less marked fiber entanglement. Consequently, the biogreases (especially OECPT) may easily release oil into the contact area, thus facilitating formation of a fluid film and decreasing the friction coefficient in the mixed lubrication region as a result (Fig. 4b). Wear Wear was assessed in stationary friction tests performed under conditions typical of the mixed lubrication regime (viz., a 20 N normal load and a 10 rpm rotational speed) for 10 min. Fig. 5 shows the average diameters of the wear scars formed in the steel plates and the stationary friction coefficient as a function of temperature, epoxy index and chemical structure of the epoxy compound. As can be seen, the friction coefficient at 95 C was ca. 0.12 for all biogreases. This value is quite similar to that for castor oil under the same conditions as a result of the effect of viscosity of base oil prevailing at high temperatures. As noted earlier, the friction coefficient was lower at 25 C than it was at 95 C probably due to the increased viscosity of the fluid film at the lower temperature shifting formation of the film to higher sliding speeds as the temperature was raised [23]. Overall, using epoxidized cellulose pulp as thickener substantially reduced wear scar diameter (particularly at 25 C), which is consistent with the markedly decreased friction coefficients obtained in the mixed lubrication region. As can be seen from Fig. 5, the wear scar diameters obtained by using biogreases containing cellulose pulp with low epoxy index or epoxidized with aromatic epoxy compounds were smaller than those obtained with cellulose pulp prepared with aliphatic epoxy compounds or having high epoxy indices (especially at 25 C). Regarding the nature of the epoxide compound, the biogrease OECPR resulted in a considerably smaller wear scar diameter (370 mm) than OECPT (430 mm).
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The epoxy index had a less marked influence; thus, the wear scar diameter obtained with sample OECPN-1 was 40 mm smaller than that obtained with OECPN-20. At 95 C, however, the diameter was quite similar for all samples (430–443 mm) as a consequence of the above-described oil bleeding effect, i.e. the influence of the base oil viscosity. In order to better understand the wearing process when using epoxidized cellulose pulp-based biogreases, the topography of the scars was examined by SEM and EDS (see Tables 2 and 3). As can be seen, all wear scars were relatively narrow and shallow linear grooves in the sliding direction suggestive of the occurrence of abrasive wear. In particular, the sample to which the biogrease OECPN-20 was applied exhibited severe abrasion on the track suggesting inadequate formation of a tribofilm or a low fluid film strength [30]. Similarly, severe abrasion track lines, and signs of micropitting, were found with castor oil alone. The EDS technique revealed the presence of higher amounts of oxygen in the wear scars formed with the biogreases, which suggests the formation of an oxide layer as a consequence of ECPs fibers reaching the ball-ondisc contact area. Deposition of a tribo-oxide film essentially consisting of Fe2O3 or Cr2O3 on the contact surface was previously found to slightly decrease the amounts of Fe and Cr [31]. The tribochemical reactions involved in the process are favored by a high epoxy index (especially at high temperatures). In this respect, the amount of oxygen found in the scars formed at 95 C was substantially greater with the biogreases containing cellulose pulp with epoxy indices higher than 2.2 mol/kg. Conclusions Overall, epoxy-modified cellulose pulp-based biogreases exhibited excellent thermal stability and no appreciable change in viscoelastic functions up to 150 C. In addition, modifying the pulp with an epoxide compound strongly altered the friction coefficient and wear relative the use of castor oil alone as lubricant (particularly at 25 C and in the low-speed region). Destruction of the thickener network by effect of shearing and an increased temperature seems to favor the release of the base oil and entrainment of the bulk grease into the contact area. The pulp samples with the higher epoxy indices were more compatible with castor oil; decreasing the friction coefficient in the mixed lubrication region. The increased compatibility also influenced wear scar diameter. Thus, the use of biogreases with lower epoxy indices yield slightly reduced wear. On the other hand, the cellulose pulps modified with aromatic epoxides used as
Fig. 5. Stationary friction coefficients obtained by applying a constant rotational speed of 10 rpm and a 20 N normal load at 25 and 95 C.
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Table 2 Elemental analysis of wear scars as a function of epoxy index at 25 and 95 C. All element contents are expressed as percentages. Castor oil
OECPN-1
OECPN-5
OECPN-20
2.369 0.940 17.018 0.200 67.981 1.336 7.864 0.363 1.736 0.052
4.656 2.949 16.409 1.512 64.842 4.561 8.276 0.467 1.662 0.372
4.246 2.769 16.137 2.057 66.030 3.466 9.239 1.236 1.584 0.226
4.656 2.949 16.409 1.512 64.842 4.561 8.276 0.467 1.662 0.372
25 C
O Cr Fe Ni Mn
0.538 0.673 17.969 1.406 68.719 1.920 7.692 0.507 1.822 0.126
2.743 0.544 16.857 0.110 67.804 0.079 7.553 0.125 1.675 0.041 95 C
O Cr Fe Ni Mn
– 18.258 0.315 71.201 0.278 7.912 0.117 1.898 0.046
1.079 0.497 16.200 1.810 69.321 1.204 9.206 1.617 1.652 0.184
Table 3 Elemental analysis of wear scars as a function of the chemical structure of the epoxide compound at 25 and 95 C. All element contents are expressed as percentages. OECPB
OECPR
OECPT
OECPN
2.091 0.570 16.415 1.085 68.934 1.289 8.256 0.404 1.603 0.171
2.912 2.105 17.415 0.509 67.762 2.841 7.791 0.352 1.682 0.098
2.680 1.028 15.643 0.441 67.708 0.908 9.198 0.377 2.540 0.043
2.408 0.340 16.293 2.061 68.011 0.902 8.711 1.011 1.757 0.385
25 C
O Cr Fe Ni Mn
2.707 2.041 16.926 0.764 67.589 2.592 7.985 0.520 1.690 0.069
1.173 1.286 17.857 0.418 69.413 2.006 7.723 0.148 1.854 0.100 95 C
O Cr Fe Ni Mn
1.866 0.666 17.035 1.108 67.998 1.382 9.114 1.074 1.781 0.674
13.200 2.006 20.066 0.540 56.218 2.030 6.854 0.181 1.607 0.071
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thickening agents were more efficient in protecting the metal surface and led to smaller wear scar diameters as a consequence. Acknowledgments The authors gratefully acknowledge cofounding of this work by MINECO-FEDER (Projects CTQ2014-56038-C3-1R) and the Andalusian Regional Government (Project TEP-1499). One of us (E. Cortés-Triviño) is additionally grateful to the Regional Government for award of a Ph.D. research grant. References [1] H.M. Mobarak, E. Niza Mohamad, H.H. Masjuki, M.A. Kalam, K.A.H. Al Mahmud, M. Habibullah, A.M. Ashraful, Renew. Sustain. Energy Rev. 33 (2014) 34, doi: http://dx.doi.org/10.1016/j.rser.2014.01.062. [2] E.R. Booser, Tribology Data Handbook: An Excellent Friction, Lubrication and Wear Resource, CRC Press, Boca Raton, FL, 1997. [3] A. Adhvaryu, S.Z. Erhan, Ind. Crops Prod. 15 (3) (2002) 247, doi:http://dx.doi. org/10.1016/S0926-6690(01)00120-0. [4] J. McNutt, Q.S. He, J. Ind. Eng. Chem. 36 (2016) 1, doi:http://dx.doi.org/10.1016/ j.jiec.2016.02.008. [5] R. Gallego, J.F. Arteaga, C. Valencia, M.J. Díaz, J.M. Franco, ACS Sustain. Chem. Eng. 3 (9) (2015) 2130, doi:http://dx.doi.org/10.1021/acssuschemeng.5b00389. [6] J.E. Martín-Alfonso, N. Núñez, C. Valencia, J.M. Franco, M.J. Díaz, J. Ind. Eng. Chem. 17 (5) (2011) 818, doi:http://dx.doi.org/10.1016/j.jiec.2011.09.003. [7] R.K. Singh, S. Pandey, R.C. Saxena, G.D. Thakre, N. Atray, S.S. Ray, J. Ind. Eng. Chem. 26 (2015) 149, doi:http://dx.doi.org/10.1016/j.jiec.2014.11.027. [8] D.S. Ogunniyi, Bioresour. Technol. 97 (9) (2006) 1086, doi:http://dx.doi.org/ 10.1016/J.BIORTECH.2005.03.028. [9] W.J. Bartz, Tribol. Int. 31 (1) (1998) 35, doi:http://dx.doi.org/10.1016/S0301679X(98)00006-1. [10] E. Cortés-Triviño, C. Valencia, M.A. Delgado, J.M. Franco, Carbohydr. Polym. 199 (2018) 563, doi:http://dx.doi.org/10.1016/J.CARBPOL.2018.07.058. [11] L. Salomonsson, G. Stang, B. Zhmud, Tribol. Trans. 50 (3) (2007) 302, doi:http:// dx.doi.org/10.1080/10402000701413471.
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Thermo-rheological and tribological properties of novel bio-lubricating greases thickened with epoxidized lignocellulosic materials E. Cortés-Triviño* , C. Valencia, M.A. Delgado, J.M. Franco Pro2TecS — Chemical Process and Product Technology Research Centre, Department of Chemical Engineering, University of Huelva, 21071 Huelva, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 November 2018 Received in revised form 29 April 2019 Accepted 21 August 2019 Available online xxx
We examined the rheological and tribological behavior of novel formulations based on castor oil and epoxidized cellulose pulp intended for use as biodegradable lubricating greases. Epoxidized cellulose pulp was found to thicken castor oil to a variable extent depending on its modification degree and the epoxide compound. Greases were subjected to small-amplitude oscillatory shear tests, evaluating the temperature-dependence of the plateau modulus. In addition, friction coefficient and wear were determined in a steel–steel ball-on-three-plates tribological configuration, at two different temperatures (25 and 95 C), generally obtaining smaller values of both parameters when using aromatic diepoxides instead of aliphatic to modify the cellulose pulp. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Bio-lubricating greases Epoxidized cellulose Frictional behavior Rheology
Introduction Biobased lubricating greases have aroused especial attention in recent years owing to their non-polluting effects on the environment [1]. Lubricating greases are semi-solid gel-like systems usually consisting of a mineral or synthetic base oil, a thickener (usually a metal soap) and an additive package that is intended to improve specific beneficial properties [2]. Most of these components are petroleum-derived products and hence obtained from finite resources; also, they can have detrimental effects on human health and ecosystems. These drawbacks have raised growing concern about the potential environmental damage caused by lubricating greases and led the scientific community to explore effective ways to replace some of their components with naturally occurring alternatives in order to develop more environmentally friendly formulations. Although the most feasible choice in this respect is replacing mineral and synthetic oils with vegetable oils [3,4], there have also been attempts at using biobased thickeners (e.g., chemically modified biopolymers [5,6]) and biobased additives (e.g., natural antioxidants or anti-wear agents [7]). The hydroxyl and epoxy groups or polyunsaturation in some vegetable oils provide special chemical functionalities that enhance lubricating properties [2]. Castor oil has been extensively investigated in this respect on the grounds of its wide availability,
* Corresponding author at: Departamento de Ingeniería Química, Campus de “El Carmen”, Universidad de Huelva, 21071 Huelva, Spain. E-mail address: [email protected] (E. Cortés-Triviño).
favorable temperature-dependence of its viscosity and optimum lubrication properties at high temperatures relative to most vegetable oils [8]. However, finding appropriate bio-thickeners for these oil media remains a major challenge since some of the required functional properties (e.g., gel-like behavior, thermal resistance, lubrication efficiency) are still unsuitable for use in completely bio-based formulations intended to replace conventional lubricating greases [9]. Isocyanate-functionalized cellulose pulp dispersions in castor oil have proved excellent candidates for this purpose [5] as they effectively mimic the rheological and tribological behavior of conventional lithium soap-based lubricating greases. Although the resulting polyurethane gel-like materials are assumed to be inert, diisocyanates are generally toxic and hazardous, which precludes their use as green ingredients in completely renewable formulations and sustainable industrial processes. This shortcoming has been addressed by chemically modifying cellulose pulp through epoxidation [10]. Epoxidized cellulose pulp gel-like dispersions in castor oil possess appropriate rheological properties and good physical stability. However, some of their functional properties (especially their tribological and rheological characteristics), and their temperature-dependence, require further investigations before these biobased formulations can be deemed suitable lubricants. The primary purpose of these materials is lubricating two moving surfaces in contact in order to reduce the resulting friction and wear. This requires the gel-like colloidal system formed by the thickener and base oil to withstand deformation when a shear stress is applied so that elastic and viscous forces can be balanced in order to avoid grease leakage and facilitate the release of base oil
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into the tribological contact area. In fact, the base oil and thickener constitute a complex rheological system [11] whose characteristics dictate whether an effective lubricant film will be formed. As a result, the efficiency of a lubricant in reducing friction is governed mainly by the viscosity of the oil and the operating conditions, whose mutual relationship is often expressed in the form of Stribeck curves. The presence of polymeric components or classical thickener particles is known to increase the thickness and viscosity of the fluid film, and hence to decrease friction coefficients and wear as compared to the base oil alone [12]. Some operational variables such as shear stress, temperature and normal load, among others, can alter the microstructural network of a lubricating grease and the viscosity of the bleeding oil, thereby decisively affecting their tribological behavior [13]. Based on the foregoing, and on the results of preliminary research into the topic [10], the main goal of this work was to assess the suitability of epoxidized cellulose pulp-based oleogels as efficient lubricating greases by examining their rheological and tribological responses as a function of temperature and the nature of the epoxidized cellulose pulp used as biogenic thickening agent.
1 mm gap). The tests were conducted in a frequency range of 0.03– 100 rad/s within the linear viscoelastic region (LVR), at selected temperatures over the range 25–150 C. Tribological characterization
Cellulose pulp (CP) from Eucalyptus globulus was kindly supplied by ENCE, S.A. (Huelva, Spain). Neopentyl glycol diglycidyl ether (NPGDGE), bisphenol A diglycidyl ether (BADGE), resorcinol diglycidyl ether (RDGE) and trimethylolpropane triglycidyl ether (TMPTGE), all from Sigma-Aldrich, were used as epoxide modifiers. Castor oil was supplied by Guinama (Valencia, Spain).
The oleogel samples were characterized for friction in a ball-onthree plates tribological cell coupled to the Physica MCR 501 rheometer. The ensemble comprised a lower measuring geometry with three 45 inclined steel plates (1.4301 AISI 304, 0.21 mm roughness, 80 HRB hardness) where the lubricant was placed and a rotating shaft holding a 6.35 mm diameter steel bearing ball (1.4401 grade 100 AISI 316). Details of the cell design, and the procedure used to calculate normal forces and sliding velocities for this geometry, can be found elsewhere [14]. Pure sliding condition was selected as the most adverse conditions to evaluate both friction and wear. The friction coefficient was monitored at 25 and 95 C by applying a rotational speed sweep from 0.01 to 1000 rpm at a normal load of 20 N. The stationary friction coefficient was determined at the same temperatures by setting a normal force of 20 N and a constant rotational speed of 10 rpm, within the mixed friction regime, and till reaching a steady state value (for 10 min). Each test was replicated 5 times, using fresh lubricating grease in each run. The topography of the wear scar surface left in the steel plates by the constant rotational speed tests was examined by scanning electron microscopy (SEM) on a JXA-8200 SuperProbe (JEOL) microscope. Samples were also subjected to electron probe microanalysis (EPMA) at 15 keV in order to determine the elementary composition of the worn surfaces by energy-dispersive spectroscopy (EDS). All measurements were made in triplicate.
Preparation of epoxidized cellulose pulp-based oleogels
Results and discussion
The starting cellulose pulp was modified with various di-or triepoxides as described elsewhere [10]. Table 1 shows the input CP/ epoxide compound weight ratios used to obtain the differently epoxidized CP samples (ECPs) for thickening the castor oil, as well as the resulting epoxidation degree (determined according to ISO 3001:1999(E)). The ECP samples were subsequently dispersed in castor oil [10], by using a 5% (w/w) concentration in NPGDGEbased samples and a 4% (w/w) in those epoxidized with the other epoxides. (Table 1). The chemical interaction between residual epoxy groups in the ECPs and hydroxyl groups in castor oil led to physically stable oleogels similar to lubricating greases in appearance and consistency.
Thermorheological response
Experimental section Materials
Rheological measurements Small-amplitude oscillatory shear (SAOS) tests were performed on a Physica MCR 501 controlled-stress rheometer from Anton Paar GmbH (Graz, Austria) equipped with a Peltier temperature control system using a serrated plate-plate geometry (25 mm diameter,
All epoxidized cellulose pulp dispersions in castor oil exhibited a typical gel-like behavior closely resembling the rheological response of conventional lubricating greases [15,16]; thus, their storage modulus (G0 ) exceeded their loss modulus (G00 ) throughout the frequency and temperature ranges examined. By way of example, Fig. 1 illustrates the frequency-dependence of the storage and loss moduli for a selected epoxidized cellulose pulp-based oleogel (OECPN-5) over the temperature range 25–150 C. As can be seen, the mechanical spectra exhibited a minimum in G00 and a power-law dependence of G0 on the frequency, which is typical of highly structured colloidal particle gels [17]. Moreover, raising the temperature to 150 C decreased the storage and loss moduli of the oleogels but only slightly. This result contrasts with the typical thermorheological behavior of conventional metal soap-thickened greases, whose SAOS functions decrease dramatically above a critical temperature (e.g., ca. 110 C for lithium greases) as a consequence of structural changes in the thickener network [18].
Table 1 Preparation conditions used to obtain the biogreases. Oleogel code OECPN-1 OECPN-2 OECPN-5 OECPN-10 OECPN-20 OECPB OECPR OECPT OECPN
Epoxy compound NPGDGE
BADGE RDGE TMPTGE NPGDGE
CP/epoxy wt. ratio
Epoxy index (mol/kg)
Thickener wt%
1/1 1/2 1/5 1/10 1/20 1/2 1/2 1/5 1/5
0.6 1.6 2.2 3.6 3.8 2.2 2.2 2.2 2.2
5 5 5 5 5 4 4 4 4
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greases obtained with an aromatic epoxidized cellulose pulpbased thickener [10]. By contrast, the epoxy index seemingly had no appreciable influence on Ea. Frictional behavior
Fig. 1. Variation of the storage modulus (G0 , closed symbols) and loss modulus (G00 , open symbols) for sample OECPN-5 as function of temperature.
For easier understanding of the temperature dependence, Fig. 2 shows the variation of the plateau modulus (GN0, defined elsewhere [19]) with the reciprocal temperature for all epoxidized cellulose pulp-based lubricating greases studied. As can be seen, the variation conformed to the following Arrhenius-type equation: Ea
G0N ¼ AeRT
ð1Þ
where A is the pre-exponential factor, Ea the activation energy, R the gas constant (8.314 J/mol K) and T the absolute temperature. As can be seen, the activation energy, obtained by fitting the curve for each oleogel to Eq. (1) was noticeably low (1.5–5.3 kJ/mol) throughout the temperature range, which suggests a weak temperature-dependence of all samples resulting viscoelastic moduli barely affected by the temperature. Overall, the Ea values are similar to those for commercial lubricating greases in the low temperature range (i.e., below the critical temperature). However, they are much lower than the values typically found above the critical temperature, which is usually dependent on the type of metal soap used as thickener [20]. Although the viscoelastic functions were only slightly influenced by temperature in all samples, using aromatic difunctional epoxides to modify the cellulose pulp led to an increased thermal susceptibility as reflected in also increased activation energy values (Ea = 5.1– 5.3 kJ/mol). This small increase in activation energy may have resulted from increased microstructural interconnection in the
Fig. 3 plots the evolution of the friction coefficient with rotational speed for sample OECPN-5 as obtained by using the tribocell with a 20 N normal load at 25 and 95 C. The curves for the oleogel are compared with the frictional response of the base oil. As can be seen, the epoxidized thickener has a substantial impact on frictional behavior, which was also dependent on temperature and the sliding velocity [21]. Adding ECP to castor oil reduced the friction coefficient in the thickener-dominated region (i.e., under the boundary and mixed lubrication regimes), but mainly at low temperatures. This behavior in the low-speed region may be ascribed to the influence of residual epoxidized cellulose fibers, which may be deposited onto the contact area at low rotational speeds, reducing the asperity contact [22,23]. However, the friction coefficient for the biogreases at the beginning of the full fluid film lubrication regime seems to be slightly higher than that for castor oil as a result of the much higher bulk viscosity. Consistent with previous results for metal soap- and polymerbased lubricating greases [22], the friction curves at 95 C for all lubricants, including castor oil, were largely bell-shaped between the boundary and mixed lubrication regions. At very low sliding velocities, the oleogels had much lower friction coefficients than the base oil, so the friction curves approached one another and eventually converged on a near-identical maximum value as the sliding velocity was increased. At 25 C, however, the friction coefficient levelled off at low sliding velocities and then decreased to a minimum value in the mixed lubrication region. Thus, the curves obtained at this temperature exhibit the typical evolution of the Stribeck curves, comprising the three characteristic lubrication regions (boundary, mixed and hydrodynamic), whereas those obtained at higher temperatures only exhibited the boundary region. Because all tribological tests were done under identical conditions as regards surface geometry, roughness and material combination, the friction coefficient may be plotted against Hersey’s number to obtain typical Stribeck curves. This parameter is used to normalize the abscissa of a Stribeck curve [24] in order to consider the combined influence of lubricant viscosity (h), rotational speed (V) and normal force (FN) on journal bearings. For the configuration used here and other similar geometries [24], the following modification of the dimensionless Hersey number, i.e. the so-called “hydrodynamic Stribeck parameter”, must be
Fig. 2. Variation of the plateau modulus (symbols) with temperature and fitting to the Arrhenius equation (lines) with their corresponding activation energy values.
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Fig. 3. Variation of the friction coefficient with the sliding velocity for a selected biogrease (OECPN-5) and the base (castor) oil upon application of a 20 N normal load at 25 and 95 C.
defined: S¼
h us FN
ð2Þ
having dimensions of [L1], where us is the sliding velocity. At a constant normal load and velocity, the biolubricant viscosity seems to strongly influence the tribological behavior of greases. Thus, a high viscosity favors hydrodynamic lubrication, whereas a very low viscosity results in friction coefficients in the boundary lubrication region [25]. Although the previous relation was developed for Newtonian lubricating oils, it is also useful to obtain information about the tribological behavior of lubricating greases [26]. This requires expressing the viscosity in terms of the base oil and thickener. However, as found elsewhere [27], subjecting grease to extreme shearing for a long time (e.g., in tribological tests) results in considerable microstructural degradation that leads to the separation between oil and thickener, i.e., the “oil bleeding” effect. Therefore, with non-Newtonian systems, the viscosity of the base oil can be, in principle, directly introduced in the expression for the hydrodynamic Stribeck parameter [26], as follows, S¼
hBO us FN
ð3Þ
where hBO is the Newtonian viscosity of the base oil. Parameter S allowed us to directly compare the friction coefficients for epoxidized cellulose pulp-based biogreases at 25 and 95 C simply by substituting the viscosity of castor oil at those temperatures. No
piezoviscosity coefficient (α) the equation since the base oil used was always the same [28]. Fig. 4 shows the master Stribeck curves obtained by overlapping the friction coefficients for epoxidized cellulose pulp-based biogreases as a function of the epoxy index, chemical structure of the epoxy compound and temperature. Most frictional curves obtained at both temperatures roughly overlapped in the manner of Stribeck curves, thus exhibiting the typical lubrication regions. However, this overlapping was not clearly observed in the hydrodynamic region, i.e. at high sliding velocities, where the bulk grease viscosity, and hence the type of thickener used, seem to have a decisive influence on the frictional behavior of the greases. Also, there were some differences in overlap in the mixed lubrication region at 25 C, especially between cellulose pulp samples epoxidized with different compounds (Fig. 4b). Such differences indicate that using the viscosity of the base oil, hBO, in the expression for the Stribeck parameter (Eq. (2)) does not allow the frictional behavior of lubricating greases to be accurately described. As shown below, the rate of grease flow into the ball-ondisc contact area increased in the mixed lubrication region, where entrainment of epoxidized cellulose particles could considerably decreased the friction coefficient (especially at 25 C). To what extent the friction coefficient is reduced in the mixed lubrication region depends on the particular thickener (specifically, on the epoxy index and chemical structure). At 95 C, however, the viscosity of castor oil prevailed in all formulations (due to the oil bleeding effect) and all curves overlapped irrespective of the particular thickener, probably as a result of temperature-induced softening of the oleogels reducing the differences in friction coefficient among the biogreases. It should be noted that the friction coefficients for the biogreases exceeded that for the base oil with full fluid film lubrication and thus depended on the bulk rheological behavior of the biogreases [22,23]. Therefore, the mechanism behind full fluid film formation appears markedly influenced by both the epoxy index and the chemical structure of the thickener. Epoxidation of lignocellulosic materials was previously found to increase their compatibility with castor oil [29]. Thus, the higher the epoxy index was, the better was solvency between the epoxidized cellulose pulp and castor oil. However, continuous over-rolling during tribological tests results in substantial shearinduced structural breakdown and facilitates release of the base oil, thereby favoring formation of a fluid film [27]. Nevertheless, the biogrease OECPN-20, with a high epoxy index, developed an extended crosslinked network that may hinder the release of oil at the lower sliding speeds. Once a steady lubricant film has formed, the friction coefficient seems to be considerably decreased as a
Fig. 4. Plot of the Stribeck parameter against the friction coefficients for the biogreases.
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consequence of the low viscosity and more effective replenishment of bulk grease in the contact area, which may have extended the mixed lubrication region to higher sliding speeds (Fig. 4a). Consequently, improved compatibility between ECPs and castor oil may delay formation of the full fluid film until high sliding velocities are reached, thus shifting the transition point from the mixed lubrication region to the hydrodynamic region to the right [26]. The influence of the nature of the epoxy compound was assessed by examining the friction curves obtained by using different oleogels with an identical epoxy index as lubricating greases (Fig. 4b). As noted earlier, cellulose pulp epoxidized with an aliphatic compound forms oleogels with less marked fiber entanglement. Consequently, the biogreases (especially OECPT) may easily release oil into the contact area, thus facilitating formation of a fluid film and decreasing the friction coefficient in the mixed lubrication region as a result (Fig. 4b). Wear Wear was assessed in stationary friction tests performed under conditions typical of the mixed lubrication regime (viz., a 20 N normal load and a 10 rpm rotational speed) for 10 min. Fig. 5 shows the average diameters of the wear scars formed in the steel plates and the stationary friction coefficient as a function of temperature, epoxy index and chemical structure of the epoxy compound. As can be seen, the friction coefficient at 95 C was ca. 0.12 for all biogreases. This value is quite similar to that for castor oil under the same conditions as a result of the effect of viscosity of base oil prevailing at high temperatures. As noted earlier, the friction coefficient was lower at 25 C than it was at 95 C probably due to the increased viscosity of the fluid film at the lower temperature shifting formation of the film to higher sliding speeds as the temperature was raised [23]. Overall, using epoxidized cellulose pulp as thickener substantially reduced wear scar diameter (particularly at 25 C), which is consistent with the markedly decreased friction coefficients obtained in the mixed lubrication region. As can be seen from Fig. 5, the wear scar diameters obtained by using biogreases containing cellulose pulp with low epoxy index or epoxidized with aromatic epoxy compounds were smaller than those obtained with cellulose pulp prepared with aliphatic epoxy compounds or having high epoxy indices (especially at 25 C). Regarding the nature of the epoxide compound, the biogrease OECPR resulted in a considerably smaller wear scar diameter (370 mm) than OECPT (430 mm).
5
The epoxy index had a less marked influence; thus, the wear scar diameter obtained with sample OECPN-1 was 40 mm smaller than that obtained with OECPN-20. At 95 C, however, the diameter was quite similar for all samples (430–443 mm) as a consequence of the above-described oil bleeding effect, i.e. the influence of the base oil viscosity. In order to better understand the wearing process when using epoxidized cellulose pulp-based biogreases, the topography of the scars was examined by SEM and EDS (see Tables 2 and 3). As can be seen, all wear scars were relatively narrow and shallow linear grooves in the sliding direction suggestive of the occurrence of abrasive wear. In particular, the sample to which the biogrease OECPN-20 was applied exhibited severe abrasion on the track suggesting inadequate formation of a tribofilm or a low fluid film strength [30]. Similarly, severe abrasion track lines, and signs of micropitting, were found with castor oil alone. The EDS technique revealed the presence of higher amounts of oxygen in the wear scars formed with the biogreases, which suggests the formation of an oxide layer as a consequence of ECPs fibers reaching the ball-ondisc contact area. Deposition of a tribo-oxide film essentially consisting of Fe2O3 or Cr2O3 on the contact surface was previously found to slightly decrease the amounts of Fe and Cr [31]. The tribochemical reactions involved in the process are favored by a high epoxy index (especially at high temperatures). In this respect, the amount of oxygen found in the scars formed at 95 C was substantially greater with the biogreases containing cellulose pulp with epoxy indices higher than 2.2 mol/kg. Conclusions Overall, epoxy-modified cellulose pulp-based biogreases exhibited excellent thermal stability and no appreciable change in viscoelastic functions up to 150 C. In addition, modifying the pulp with an epoxide compound strongly altered the friction coefficient and wear relative the use of castor oil alone as lubricant (particularly at 25 C and in the low-speed region). Destruction of the thickener network by effect of shearing and an increased temperature seems to favor the release of the base oil and entrainment of the bulk grease into the contact area. The pulp samples with the higher epoxy indices were more compatible with castor oil; decreasing the friction coefficient in the mixed lubrication region. The increased compatibility also influenced wear scar diameter. Thus, the use of biogreases with lower epoxy indices yield slightly reduced wear. On the other hand, the cellulose pulps modified with aromatic epoxides used as
Fig. 5. Stationary friction coefficients obtained by applying a constant rotational speed of 10 rpm and a 20 N normal load at 25 and 95 C.
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Table 2 Elemental analysis of wear scars as a function of epoxy index at 25 and 95 C. All element contents are expressed as percentages. Castor oil
OECPN-1
OECPN-5
OECPN-20
2.369 0.940 17.018 0.200 67.981 1.336 7.864 0.363 1.736 0.052
4.656 2.949 16.409 1.512 64.842 4.561 8.276 0.467 1.662 0.372
4.246 2.769 16.137 2.057 66.030 3.466 9.239 1.236 1.584 0.226
4.656 2.949 16.409 1.512 64.842 4.561 8.276 0.467 1.662 0.372
25 C
O Cr Fe Ni Mn
0.538 0.673 17.969 1.406 68.719 1.920 7.692 0.507 1.822 0.126
2.743 0.544 16.857 0.110 67.804 0.079 7.553 0.125 1.675 0.041 95 C
O Cr Fe Ni Mn
– 18.258 0.315 71.201 0.278 7.912 0.117 1.898 0.046
1.079 0.497 16.200 1.810 69.321 1.204 9.206 1.617 1.652 0.184
Table 3 Elemental analysis of wear scars as a function of the chemical structure of the epoxide compound at 25 and 95 C. All element contents are expressed as percentages. OECPB
OECPR
OECPT
OECPN
2.091 0.570 16.415 1.085 68.934 1.289 8.256 0.404 1.603 0.171
2.912 2.105 17.415 0.509 67.762 2.841 7.791 0.352 1.682 0.098
2.680 1.028 15.643 0.441 67.708 0.908 9.198 0.377 2.540 0.043
2.408 0.340 16.293 2.061 68.011 0.902 8.711 1.011 1.757 0.385
25 C
O Cr Fe Ni Mn
2.707 2.041 16.926 0.764 67.589 2.592 7.985 0.520 1.690 0.069
1.173 1.286 17.857 0.418 69.413 2.006 7.723 0.148 1.854 0.100 95 C
O Cr Fe Ni Mn
1.866 0.666 17.035 1.108 67.998 1.382 9.114 1.074 1.781 0.674
13.200 2.006 20.066 0.540 56.218 2.030 6.854 0.181 1.607 0.071
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