Recycled and virgin LDPE as rheology modifiers of lithium lubricating greases: a comparative study
In this work, a new application for recycled low-density polyethylene (LDPE), as rheology modifier of standard lithium lubricating grease formulations, was studied. The effectiveness of this additive was compared with that achieved with a virgin LDPE. With this aim, both types of polymers were added to the formulation during the manufacturing process of greases, following the same standard protocol, to reinforce the role of the thickening agent, the lithium 12-hidroxystearate. The effect that both lithium soap and LDPE concentrations exert on the rheology of lubricating grease formulations and its relationship with grease microstructure were discussed. Lubricating greases were rheologically characterized through small-amplitude oscillatory shear and viscous flow measurements. In addition to these, scanning electron microscopy observations and mechanical stability tests were also carried out. In all cases, an increase in soap concentration yields higher values of apparent viscosity and linear viscoelasticity functions. On the other hand, the values of the Theological functions obtained for recycled LDPE-based lubricating greases are, in general, higher than those obtained for virgin LDPE-based grease formulations. However, the structural skeleton developed in greases containing recycled LDPE demonstrates less resistance to severe working conditions, showing lower mechanical stability than virgin LDPE-based grease formulations. POLYM. ENG. SCI., 48:1112-1119, 2008. [c] 2008 Society of Plastics Engineers
Lubricating greases are generally considered as semisolid colloidal dispersions, owing their consistency to a gel-forming network. They consist essentially of a thickening agent dispersed in a viscous liquid medium. Fatty acid soaps of lithium, calcium, sodium, aluminum, or barium are most commonly used as thickeners. The thickener is primarily a bodying agent, i.e. immobilizes the oil and provides varying resistance to flow (1). In most cases, the liquid medium is a lubricating oil that provides lubricant features to the system.
The performance of lubricating greases depends on the nature of its components and the microstructure achieved during processing (2). Suitable rheological and micro-structural characteristics may be obtained from both an adequate selection of ingredients and process optimization, as previously reported (2), (3). The inclusion of polymers in grease formulations has been a common practice for many years. Polymers have played a significant role as additives in greases for modification of some performance characteristics such as dropping point, appearance, structure, tackiness, water resistance, and bleed. Use of polymers may have become less prevalent over the years since the fatty acid soaps evolved as the primary thickeners for many grease manufactures. However, as the interest for improved performance and multiservice greases has increased during the last two decades, polymers are being used more frequently (4). In general, polymers are often used as supplements to other types of thickening agents, to improve the adhesion and cohesion characteristics of lubricating greases (5). Besides these performance characteristics, the addition of polymers to grease formulations largely influences the rheological behavior of these products (6).
The rheology of greases is important not only for preventing loss of lubricant or reinforcing sealing properties, but also under working conditions. As pointed out by several authors (7), (8), the lubricating performance in a tribological contact is affected by the sampling mechanism of lubricant from the sides of the contact, which evidently depends on the flow properties of the grease. In this sense, different concentrations of polymeric additives can be used to adjust the required rheological properties for each application.
On the other hand, the mechanical recycling of post-consumer plastics remains one of the preferred recycling options for ecological and energetic reasons, as far as this way is economically profitable. Household plastic wastes comprise very dissimilar generic polymers. The polyolefins are the main components being about 60-70 wt [percent], while other polymers, such as PS, PET, and PVC, make up the remaining part of the composition, with possibly minor quantities of polyamide, polycarbonate, acrylic polymers (9-11). Low-density polyethylene (LDPE) represents an important weight percentage of the polyolefins found in waste streams, but this polymer alone is not really interesting for particular applications because it exhibits average mechanical properties, which are moreover influenced by the aging of the product. More frequently, LDPE wastes come from bags and pallet covers, being recycled as garbage bags, which is a very limited application. New outlets for this kind of LDPE wastes can be developed if their limited mechanical properties can be strengthened by adding to other materials (11-13). Thus, the applicability of such polymers in product development may imply an incentive to the industry, since they represent a very cheap source of new materials besides its environmental benefits.
The main objective of this research was to study and compare the mechanical characteristics of lithium lubricating greases containing virgin or recycled LDPE as rheological modifier.
12-hydroxystearic acid, anhydrous lithium hydroxide, and a naphtenic mineral lubricating oil (density at 20[degrees]C: 916 [kg/m.sup.3]; kinematic viscosity at 40[degrees]C: 115 cSt) were used as received to prepare lithium 12-hydroxystearate lubricating greases. All the components were kindly supplied by Verkol, S.A. (Spain). Virgin and recycled LDPE samples were used as additive of lubricating greases. Virgin LDPE (Dow Chemical, Ref. 302R), with density 0.924 g/[cm.sup.3] and melt flow index 0.8 g/10 min (190 C/2.16 kg), was used as received. Recycled LDPE from plastic bags, with similar density (0.922 g/[cm.sup.3]) and melt flow index (0.8 g/10 min), was supplied by Eslava Plasticos, S.A (Spain).
Manufacture of Lubricating Greases
The process was performed in a stirred batch reactor (600 g), using an anchor impeller geometry and applying a rotational speed of 60 rpm Processing details and procedure have been extensively described elsewhere (2), (3). Once the saponification reaction was completed, the mixture was heated up to a maximum temperature of 180[degrees]C, to induce the phase transition of soap crystallites into a waxy phase and complete the dehydration process. Afterwards the mixture was cooled down to room temperature. In this step, two batches of oil, at room temperature, were added to help cooling. Polymers were previously placed in one of these batches of oil and held at 125[degrees]C for a period of 12 h. This blend was added to the grease formulation when the mixture was at 180[degrees]C approximately. A final homogenization treatment (rotational speed: 8800 rpm; homogenization time: 15 min), using a rotor-stator turbine (Ultra Turrax T-50, Ika, Staufen, Germany), was applied at room temperature.
Lithium soap and virgin or recycled LDPE concentrations in the final product ranged from 4.0 to 14.0[percent] (w/w) and 0-5.0[percent], respectively.
The rheological measurements were carried out in both controlled-stress (Gemini BOHLIN, UK) and controlled-strain (Rheometric ARES, UK) rheometers, using a plate-plate geometry (25 mm diameter, 1 mm gap) with rough surfaces. Small-amplitude oscillatory shear (SAOS) measurements, inside the linear viscoelasticity regime, were performed in a frequency range comprised between [10.sup.-2] and [10.sup.2] rad/s at 25[degrees]C. Strain sweep tests, at the frequency of 1 Hz, were previously performed on each sample to determine the linear viscoelasticity region. Viscous flow measurements were performed at 25[degrees]C, in a shear rate range of [10.sup.-4]-[10.sup.2] [s.sup.-1]. A serrated plate-plate geometry was used, to eliminate the wall-slip effects usually observed during the flow of these materials [14, 15]. All the samples had the same recent past thermal history. At least two replicates were performed on fresh samples.
Differential scanning calorimetry (DSC) measurements were performed with a Q-100 TA instrument, using 5-10 mg samples sealed in hermetic aluminum pans. A heating rate of 5[degrees]C/min, inside a maximum temperature range of -85 to 220[degrees]C, was applied. The sample was purged with nitrogen at a flow rate of 50 ml/min. Calibrations of temperature and enthalpy were performed with standard Indium using the thermal software version 4.0.
Both unworked and worked penetration indexes of greases were determined according to the ASTM D1403 standard, using the Seta Universal Penetrometer, model 17000-2 (Stanhope-Seta, UK), with a one-quarter cone geometry. The one-quarter scale penetration values were converted into the equivalent full-scale cone penetration values following the ASTM D217 standard. The samples were worked in a Roll Stability Tester, model 19400-3 (Stanhope-Seta, UK), according to the ASTM D1831 standard.
Morphological observations with a Scanning electronic microscopy (SEM), model JSM-5410 (JEOL, Japan), were conducted at 15 kV. A small amount of each sample was immersed, for 90 min, in hexane to extract the oil. This operation was repeated until oil extraction was completed. Afterwards the sample was dried at room temperature. Finally the samples were coated with gold. Micrographs, at different magnifications (150X to 10,000X), were taken on several samples.
RESULTS AND DISCUSSION
Differential Scanning Calorimetry Measurements
Figures 1 and 2 show heat flow curves for the virgin and recycled polymers used as additives and selected polymer-modified lubricating greases, respectively. As expected, virgin LDPE heat flow curve presents only one main peak at 112[degrees]C, corresponding to its melting point. On the contrary, recycled LDPE shows two main melting peaks (at about 111 and 124[degrees]C), suggesting that this recycled material consists of a blend of polymers. As indicated by Dintcheva et al. (16), these two peaks evidence the presence of LDPE (melting temperature close to 110[degrees]C) and another linear low density polyethylene (LLDPE) (melting temperature close to 120[degrees]C). It is well known that LLDPE is usually blended with LDPE to obtain films (17). Linear low density polyethylene (LLDPE), produced by copolymerization of ethylene and [alpha]-olefin contains short chain branches and unsaturation. LLDPE possesses greater tensile and tear strength and higher environmental stress cracking resistance than LDPE (18), (19).
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Heat flow curves of lubricating greases containing virgin LDPE (Fig. 2a) show two main peaks. The first one corresponds to the melting point of LDPE and the second one to the melting point of the lithium soap . On the other hand, DSC curves of lubricating greases containing recycled LDPE (Fig. 2b) present always three peaks. The main one corresponds to the melting of the lithium soap (at 201[degrees]C), while the two other peaks correspond to the melting points of the two main components of recycled LDPE. However, a shift of the polymer melting points to lower temperatures was observed for all the lubricating greases prepared using either virgin or recycled polymer, as a consequence of the increased effective volume of the polymer in the oil medium (13). This displacement of the polymer melting points seems to be independent of soap and polymer concentrations.
The lubricating grease samples studied in this article consist of fine particles of lithium soap in naphtenic oil. When a metal soap is dispersed in a mineral oil, the crystallized soap particles arrange themselves to form a characteristic microstructure depending on soap concentration, oil viscosity, and type of metal soap used, due to the balance of forces between the colloidal particles and the oil medium (3), (21). Figure 3 shows selected micrographs of lubricating grease samples containing virgin and recycled LDPE, at magnifications of 500X and 1000X, respectively. As can be observed, the polymer remains inserted in the grease matrix, which is composed by soap fibers randomly distributed within a given volume, as can be clearly appreciated at larger magnifications (3). As was shown in a previous article, polymer particles seem to affect the size of soap fibers, and also the hollow spaces volume among fibers where oil is trapped, being this structure spongier than that shown by the polymer-free grease (20). In the case of lubricating greases manufactured with virgin LDPE, the polymer appears as a semi-continuous layer over the soap fibers showing a low penetration level in the grease matrix. On the contrary, as it is shown in Fig. 3b, when the recycled LDPE is used in the manufacture of the lubricating grease, the polymer appears more uniformly distributed in the form of granules.
Figures 4a and 5a show the mechanical spectra, in the linear viscoelasticity range, of lubricating greases containing 2.5 wt [percent] of recycled and virgin LDPE, respectively, as a function of soap concentration. As can be observed, the linear viscoelastic behavior is qualitatively similar for all the greases studied, and also to that found with other commercial lithium lubricating greases (22). The evolution of the storage, G’, and loss, G”, moduli with frequency for these lubricating greases, inside the linear viscoelastic range, that is higher values of the storage modulus and a minimum in the loss modulus at intermediate frequencies, is characteristic of polymeric systems with physical entanglements and supports the idea that lubricating greases are highly structured systems, as has been otherwise shown in Fig. 3. In addition to this similar evolution of the SAOS functions with frequency, for both recycled and virgin LDPE, independently of the thickener concentration, the values of G’ and G” increase with soap concentration in the same manner, which generally yields rather similar values of the loss tangent (Figs. 4b and 5b). This experimental evidence indicates that the relative elastic characteristics are quite alike for all the soap concentrations evaluated. This influence of soap concentration on the SAOS response is very similar to that previously reported for polymer-free grease formulations containing lithium soap as thickener (3).
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Similar behavior can be observed in Figs. 6a and 7a, which show the evolution of the linear viscoelastic functions with frequency, as a function of recycled and virgin LDPE concentration, for lubricating greases containing 9[percent] lithium soap. Both recycled and virgin LDPE additions clearly influence the values of SAOS functions, especially at high concentrations. Moreover, the presence of LDPE in the formulation modifies the values of the loss tangent. As can be observed in Figs. 6b and 7b, the polymer-free formulation always shows the lowest values of the loss tangent, which means a more relative elastic response in spite of having lower values of G’ and G”. In the case of greases containing recycled LDPE (Fig. 6b), the relative elastic response of greases decreases as LDPE concentration increases. These results could be explained taking into account the effect of soap and polymer concentrations on the microstructure of lubricating greases. Thus, an increase in soap concentration results in a stronger structural network as a result of higher soap particle density and more entangled fibers (3), yielding larger values of both linear viscoelastic moduli. On the other hand, the role of the polymer in the formulation is twofold, acting as a filler in the entangled network and, consequently, increasing the interactions among crystallized soap fibers and the polymer itself, and modifying the base oil viscosity by increasing the polymer hydrodynamic volume in contact with the associated oil. This fact makes more important the effect of polymer on G” than on G’, yielding a decrease in the values of the loss tangent.
[FIGURE 7 OMITTED]
The plateau modulus, [G.sub.N.sup.O], which is a characteristic parameter of the plateau region, can be used as a reference parameter to evaluate the influences of soap and LDPE on grease rheology. [G.sub.N.sup.O] is defined for polymers as the extrapolation of the contribution of the entanglements to G’ at high frequencies (23) and can be easily estimated through a short-cut method from loss tangent data as previously described (6). Figure 8 shows the values of [G.sub.N.sup.O], as a function of soap and polymer concentrations, for lubricating greases containing recycled or virgin LDPE. As can be observed, [G.sub.N.sup.O] always increases with both soap and LDPE concentrations. Moreover, the values of SAOS functions obtained for lubricating greases containing recycled LDPE are, in general, higher than those obtained for lubricating greases prepared using virgin LDPE as additive (see also Fig. 8). This increase in the plateau modulus values do not seem to be influenced by soap concentration. On the contrary, larger differences between [G.sub.N.sup.O] values of greases containing virgin or recycled LDPE are noticed as polymer concentration increases (Fig. 8b). These results could be explained taking into account that, as has been previously discussed from the DSC analysis carried out on samples of this polymer (see Fig. 1), recycled LDPE consists of a blend of polymers (LLDPE and LDPE). As has been previously reported [18, 24], LDPE blended with LLDPE improves the mechanical properties and resistance of this polymer, i.e. increased viscosity and elasticity.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
From a rheological point of view, the development of a grease formulation would need the characterization of both linear viscoelastic and viscous flow responses. Figure 9 shows typical viscous flow curves for selected lithium lubricating greases. As has been previously reported (3), (20), the Sisko model, which takes into account both the shear-thinning behavior in a wide range of shear rates and the tendency to a high-shear rate limiting viscosity, fits the flow behavior of greases fairly well ([R.sup.2] > 0.995):
[eta]=m[[gamma].sup.n-1] + [[eta].sub.[infinity]] (1)
where “m” is a parameter related to the consistency of the sample, “n” is the slope of the shear-thinning region, and n( is the high-shear-rate-limiting viscosity. In the shear rate range studied, n( must be considered a not significant fitting parameter only included to fit the slight tendency of the flow curve to reach a high-shear rate Newtonian region (n(= 0.1-0.2 Pa s).
Figure 10 shows the values of the consistency index, m, for the lubricating grease samples studied, as a function of soap and LDPE concentrations. As can be observed, m increases with both variables. However, while soap concentration exerts a rather similar influence on the above-mentioned parameter, independently of the type of LDPE used as additive, the effect of LDPE content is much more dramatic in the case of lubricating greases prepared with recycled LDPE, especially at polymer concentrations higher than 1.25 wt %. On the other hand, the values of the flow index, n, are quite close to zero for all the samples studied (n= 0.005-0.01), which is representative of the typical yielding behavior shown by these materials (2), (25), This behavior is characterized by a decay of several decades in viscosity with a very small increment in shear stress. Some flow problems derived from this yielding behavior, especially at high temperatures, where discussed elsewhere (26),
[FIGURE 10 OMITTED]
The comparison between apparent and complex viscosities, obtained from viscous flow and linear viscoelastic measurements, respectively, the so-called deviation of the Cox-Merz rule, provides a quantification of the shear-induced structural breakdown of the lubricating greases studied. Figure 9 illustrates this comparison for selected formulations. Taking into account that the power-law decrease with shear rate/frequency is rather similar for both rheological functions, the relative deviation of the Cox-Merz rule can be obtained, independently of the shear rate or frequency studied, as follows:
[[eta].sub.rel] (%) = [eta]* – [eta]/[eta]*.sub.[gamma]=[omega] x 100 (2)
where [eta]* and n are the complex and apparent viscosities, respectively; irrespective of the value of frequency/shear rate selected. The results obtained from this comparison, according to Eq. 2, are shown in Tables 1 and 2, as a function of soap and polymer concentrations.
In general, these values indicate a significant degree of structural breakdown for all the lubricating greases manufactured (see Tables i and 2), although the shear-induced structural breakdown seems to be more important as soap concentration increases (see Table 1). These results are in agreement with those found for other lubricating greases containing metallic soap as the only thickener (3). On the other hand, recycled polymer addition slightly dampens this shear-induced structural breakdown (see Table 2). Moreover, virgin LDPE concentration does not exert a significant influence on [n.sub.rel] values, while, on the contrary, a slightly lower shear-induced structural breakdown was detected by increasing recycled LDPE concentration (Table 2). This fact can be again attributed to the improved mechanical properties [1.8] conveyed by LDPE/LLDPE blends to grease formulations.
TABLE 1. Relative deviation of the Cox-Merz rule (Eq. 2) for the
lubricating greases studied as a function of soap concentration.
Soap (%) Recycled LDPF. Virgin LDPE
4.0 94.6 92.8
6.5 95.7 95.9
9.0 96.4 97.1
11.5 96.9 97.4
14.0 97.4 97.5
TABLE 2. Relative deviation of the Cox-Merz rule (Eq. 2) for the
Lubricating greases studied as a function of LDPE concentration.
Polymer (%) Recycled LDPE Virgin LDPE
0 96.8 96.8
1.25 96.4 97.0
2.50 96.4 97.1
3.65 95.9 97.3
5.00 95.5 97.5
Lubricating greases should be physically and chemically stable under operating conditions, as, for instance, those found in roller bearings (27). In this work, the mechanical stability has been simulated by performing the traditional penetration measurements before and after the standardized roll-stability test. The measurements have been carried out on samples differing in both soap and LDPE concentrations. However, it is well-known that these tests only represent an approximation to classify greases according to their ability to lubricate roller bearings under real conditions (21), (27). Figures 11 and 12 show the values of penetration for unworked and worked lubricating greases, as well as the penetration increment after working, as a function of soap and polymer concentrations, respectively. Lubricating greases are usually considered stable to the continuous shear of rolling elements when its variation of penetration before and after performing the roll-stability test is close to zero. As expected, an increase in both soap and LDPE concentrations yields a decrease in the unworked penetration values, due to the increase in viscous flow parameters, i.e., consistency index. In the same way, the values of the penetration after working of greases decrease with soap concentration and polymer content, as can be observed in Figs. 11 and 12. Nevertheless, the penetration increment after working generally increases with soap and LDPE concentrations. From the mechanical stability test results obtained, it may be deduced that the highly developed structural skeleton of greases with high soap and LDPE concentrations, as the linear viscoelasticity results confirm, is, on the contrary, more sensitive to the action of the rolling track in the roll-stability test. In addition, the above-mentioned penetration increment is even more important for recycled LDPE-based greases. It is worth pointing out that the values of penetration after working do not significantly depend on the nature of the polymer used (virgin or recycled). However, lubricating greases containing recycled LDPE show higher values of the unworked penetration and, consequently, show larger positive penetration increments, more important as polymer concentration increases. This apparent loss in mechanical stability could be related to more severe changes in the microstructure of the lubricating greases containing recycled LDPE. The nature and kinetics of the structural breakdown and recovery of these materials would merit further research.
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
A comparative study has been carried out to evaluate the performance of virgin and recycled LDPE as rheology-modifiers of standard lithium lubricating greases. From the experimental results obtained, it can be deduced that both virgin and recycled LDPE could be used as modifiers of the rheological, microstructural, and mechanical properties of lithium lubricating greases. The rheological and mechanical properties of lubricating greases were analyzed as a function of polymer and soap concentrations. In all cases, and increase in soap concentration yields higher values of apparent viscosity and linear viscoelastic functions. On the other hand, the rheological functions obtained for recycled LDPE-based lubricating greases are, in general, higher than those obtained for virgin LDPE-based grease formulations. This fact can be attributed to the improved mechanical properties and resistance of recycled LDPE, composed of LLDPE and LDPE blends. However, the structural skeleton developed in greases containing recycled LDPE was more sensitive to severe working conditions, showing larger penetration increment values after working and, consequently, poorer mechanical stability.
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Correspondence to: J.M. Franco; e-mail: email@example.com
Contract grant sponsor: MEC-FEDER Program: contract grant number: CTQ2004-02706.
Published online in Wiley InterScience (www.interscience.wiley.com).
[c] 2008 Society of Plastics Engineers
J.E. Martin-Alfonso, C. Valencia, M.C. Sanchez, J.M. Franco, C. Gallegos
Departamento de Ingenieria Quimica, Faculted de Ciencias Experimentales, Universidad de Huelva, Campus de “EI Carmen”, 21071 Huelva, Spain
COPYRIGHT 2008 Society of Plastics Engineers, Inc.
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