4136
Ind. Eng. Chem. Res. 2009, 48, 4136–4144
Rheological Modification of Lubricating Greases with Recycled Polymers from Different Plastics Waste J. E. Martı´n-Alfonso, C. Valencia, M. C. Sa´nchez, J. M. Franco,* and C. Gallegos Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Experimentales, UniVersidad de HuelVa, Campus de “El Carmen”, 21071 HuelVa, Spain
There is a growing interest on the development of new materials based on recycled polymers from plastics waste, since the use of such plastics represents a low-cost source of raw material. The purpose of the present work is to evaluate, from a comparative point of view, the effect that different waste and virgin polymers, used as viscosity modifier additives, exert on the rheological properties of standard lithium lubricating greases. Grease formulations containing diverse polymers, differing in nature and/or origin, were manufactured and rheologically characterized. Particularly, the influences of the type of polymer, the molecular weight and the presence of carbon black, used as filler in recycled polymers, have been evaluated. Small-amplitude oscillatory shear (SAOS) and viscous flow measurements were carried out, as well as calorimetric and thermogravimetric analysis. In general, recycled polymers induce a more important rheological modification than virgin polymers of the same nature. Thus, the addition of some recycled polymers such as HDPE, LDPE, PP, and EVA copolymer to lithium lubricating grease significantly increases the values of the rheological parameters analyzed. The crystallinity degree, mainly dependent on the nature of the polymer, and the carbon black content of recycled polymers have been pointed out as the most highly influencing parameters on the rheology of the lubricating greases studied. However, an apparent loss in mechanical stability for lubricating greases containing recycled polymers has been found when they were submitted to a severe mechanical treatment. 1. Introduction Plastics waste recycling is one of the main methods for reducing urban and agricultural solid waste.1-5 Therefore, there is a growing interest on the development of new materials based on recycled polymers from plastics waste. Some recycled polymers with acceptable properties can be obtained from plastic residues of discarded materials found in urban waste. These materials are basically polyolefins (low density polyethylene, LDPE; linear low density polyethylene, LLDPE; high density polyethylene, HDPE; and polypropylene, PP),6 as well as polystyrene (PS). Polyolefins constitute a very versatile group of polymers with a basic structure of aliphatic hydrocarbons, which give them some important characteristics, such as hydrophobicity, low density, etc. In addition, they show a wide spectrum of mechanical properties that can be varied further by blending.7 Polyolefin blends are an important group of materials from the viewpoint of recycling, with their morphology and mechanical properties being complex issues that are governed by many factors, such as repeating units, comonomers, tacticity, molecular weight distribution, or thermomechanical history.8 Lubricating greases are semisolid colloidal dispersions of a thickening agent (usually a metal soap) in a liquid lubricant matrix (mineral or synthetic oil). Greases are the preferred lubricant in hard-to-reach places, for mechanically rubbing or dynamic systems. Many important functional properties are related to, i.e., their ability to flow under external forces, mechanical stability under shearing, thermal susceptibility, dripping and spattering, etc.9 Lubricating grease properties depend on the nature of its components and the microstructure achieved during its processing. Consequently, suitable structural and physical characteristics may be reached from a proper selection of the ingredients, as well as from a process optimiza* To whom correspondence should be addressed. Phone: +34959219995. Fax: +34959219983. E-mail:
[email protected].
tion, as was previously reported.10,11 In addition, lubricating greases usually contain some performance additives.12,13 Among them, synthetic polymers are well-known to improve some performance characteristics of lubricating greases such as dropping point, appearance, structure, tackiness, water resistance, and bleed. Besides these performance characteristics, the addition of polymers to grease formulations largely influences the rheological behavior of these products.14 In general, polymers are often used as supplements to the traditional thickening agents, in order to either decrease the thickener content by keeping constant the consistency, i.e., the NLGI grade, or increase the consistency of greases for some specific applications like high-speed bearings where leakage and sealing are particular concerns.13 The rheological modification induced by polymeric additives affects the viscoelasticity of lubricating greases, which is important for preventing loss of lubricant or reinforcing sealing properties, but also the flow properties under working conditions. From an environmental and economical point of view, the addition of plastics waste to lubricating greases presents a great interest, also taking into account that the cost is an important aspect when selecting a polymer as a modifying agent. In previous works,14,15 we have studied the effects that lithium soap and recycled low-density polyethylene (LDPE) concentrations exert on the rheology of lithium lubricating greases, as well as its relationship with grease microstructure. In addition to this, the effectiveness of recycled LDPE to modify the rheological behavior of greases was compared to that achieved with virgin LDPE. In this sense, both virgin and recycled LDPE were found to be effective additives to modify grease rheology, acting as fillers in the soap-entangled microstructure. It was apparent that recycled LDPE provides a larger rheology modification, which was attributed to the presence of LLDPE blended with LDPE.15 The purpose of the present work is to evaluate the effect that different waste and virgin polymers used as additives exert on the rheological properties of lithium
10.1021/ie801359g CCC: $40.75 2009 American Chemical Society Published on Web 03/12/2009
Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 4137 Table 1. Basic Description and Origin of the Polymeric Materials Used in This Study samples
description
recycled LDPE (1)
recycled gray-colored material, from plastic bags (Eslava Plasticos, S.A) recycled LDPE (2) recycled black-colored material, from plastic bags (Eslava Plasticos, S.A) recycled HDPE (1) recycled gray-colored material, from plastic bottles (Eslava Plasticos, S.A) recycled HDPE (2) recycled black-colored material, from plastic bottles (Eslava Plasticos, S.A) recycled EVA (1) recycled black-colored material, from greenhouse films (Egmasa, Spain), 5% vinyl acetate (VA) recycled EVA (2) recycled brown-colored material, from greenhouse films (Egmasa, Spain), 2% vinyl acetate (VA) recycled PP (1) recycled black-colored material, from unknown origin (Eslava Plasticos, S.A) recycled PP (2) recycled black-colored material, from films with inks (Eslava Plasticos, S.A) recycled PS recycled cream-colored material, from yoghurt packing (Eslava Plasticos, S.A) virgin LDPE commercial-grade LDPE (Dow Chemical) virgin HDPE commercial-grade HDPE 6006-L (Repsol YPF, Spain) virgin EVA commercial-grade (Repsol YPF, Spain), 7.33% vinyl acetate (VA) virgin PP commercial-grade Isplen PP-020 TK-26102 (Repsol YPF, Spain) virgin PS commercial-grade (Sigma Aldrich)
lubricating greases. Therefore, lubricating grease formulations containing several virgin and waste polymers from different origin were prepared and rheologically characterized. Particularly, the influences of polyolefin type, molecular weight and presence of carbon black, used as filler in recycled polymers, have been evaluated. 2. Experimental Section 2.1. Materials. 12-Hydroxystearic acid, anhydrous lithium hydroxide, and naphthenic mineral lubricating oil (density at 20 °C ) 916 kg/m3; kinematic viscosity at 40 °C ) 115 mm2/ s) were used as received to prepare lithium 12-hydroxystearate lubricating greases. These components were kindly supplied by Verkol, S.A. (Spain). On the other hand, Table 1 lists the polymers employed as additives of lubricating greases, including a basic description, and their origin. Some physical properties of the different polymer samples studied are given in Table 2. Lithium soap and polymer concentrations were fixed at 14% w/w and 2.5% w/w, respectively, in the final product, according to previous results.14 2.2. Manufacture of Lubricating Greases. The process was performed in a stirred batch reactor (600 g), using an anchor impeller geometry and a rotational speed of 60 rpm. Processing details and procedure have been extensively described elsewhere.10,16 Once the saponification reaction was completed, the mixture was heated up to a maximum temperature of 180 °C and kept constant for 60 min, in order to induce the phase transition of soap crystallites into a waxy phase and complete the dehydration process. Afterward, a controlled cooling ramp down to room temperature was imposed. During this cooling process, two batches of oil at room temperature were added. Polymer was previously dispersed in one of these oil batches and held at a temperature above the melting point, for ∼12 h, to induce polymer swelling. A homogeneous blend was obtained in all cases, which was added to the base grease formulation at the beginning of the cooling stage, at ∼180 °C, and then the polymer crystallizes together with the lithium soap. 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. As was previously shown by performing scanning electron microscopy (SEM) observations,15 in all cases polymers appeared uniformly distributed over the soap fibers in the form of granules or layers with sizes between 40 and 200 µm. No significant differences in particle size were found among the different polymer-based grease formulations studied. 2.3. Rheological Characterization. Rheological measurements were carried out in both controlled-stress (Gemini-Bohlin, U.K.) and controlled-strain (ARES, Rheometric Scientific, U.K.) rheometers, using a plate-plate (25 mm diameter, 1 mm gap) geometry. Small-amplitude oscillatory shear (SAOS) measurements, inside the linear viscoelasticity regime, were performed in a frequency range between 10-2 and 102 rad/s, at 25 °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 °C, in a shear rate range of 10-4-102 s-1. A serrated plate-plate (25 mm diameter, 1 mm gap) geometry was used in order to avoid wallslip effects usually observed during the flow of these materials.17,18 All the samples had the same recent past thermal history. At least two replicates were performed on fresh samples. 2.4. Differential Scanning Calorimetry (DSC). DSC measurements were performed with a Q-100 TA instrument, using 5-10 mg samples sealed in hermetic aluminum pans. The sample was purged with nitrogen at a flow rate of 50 mL/min. Calibration of temperatures and enthalpy was performed with standard indium, using the thermal software version 4.0. Heat flow curves were obtained by applying a temperature program of -85 to 220 °C/220 °C to -85 °C/-85 to 220 °C, with all ramps conducted at 5 °C/min for polymeric materials. The melting and glass transition temperatures and the fusion enthalpy were calculated for virgin and recycled polymer samples from the calorimetric data obtained during the second heating ramp. On the contrary, the first heating cycle was considered for lubricating greases, in order to avoid interactions between soap and polymer recrystallizations. 2.5. Thermogravimetric Analysis (TGA). TGA determinations were carried out in a Q-50 TA instrument. Approximately 10 mg of sample was heated from 30 to 600 °C at a heating rate of 10 °C/min, under a nitrogen gas total flow of 100 mL/ min. 2.6. Gel Permeation Chromatography (GPC). The molecular variables of the polymers, weight-average molecular weight, Mw, number-average molecular weight, Mn, and polydispersity, Mw/Mn, were determined by GPC, using a 150 CV Water GPC coupled to a differential refractive index instrument and viscometer. Average molecular weight data of the different polymers studied are listed in Table 2. Mw/Mn ratio ranges from 4.98 to 6.24 for virgin polymers and from 5.59 to 37.31 for recycled polymers. As expected, recycled polymers are more polydisperse than the corresponding virgin polymers, since they are usually blended materials. Only recycled PS sample shows a narrow molecular weight distribution. 2.7. Penetration and Mechanical Stability Tests. Both unworked and worked penetration indexes of lubricating greases were determined according to ASTM D1403 standard, using the Seta Universal penetrometer, model 17000-2 (Stanhope-Seta, U.K.), with a one-quarter cone geometry. The one-quarter scale penetration values were converted into the equivalent full-scale cone penetration values following ASTM D217 standard. The samples were worked in a Roll Stability tester, model 19400-3 (Stanhope-Seta, U.K.), according to ASTM D1831 standard.
4138 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 Table 2. Some Physical Properties, Average Molecular Weights, and Polydispersity Values for the Different Polymer Samples Studied sample
density (g/cm3)a
MFI (g/10 min)b
recycled LDPE (1) recycled LDPE (2) recycled HDPE (1) recycled HDPE (2) recycled EVA (1) recycled EVA (2) recycled PP (1) recycled PP (2) recycled PS virgin LDPE virgin HDPE virgin EVA virgin PP virgin PS
0.922 0.930 0.950 0.964 0.926 0.924 0.942 0.910 1.037 0.924 0.957 0.926 0.902 1.047
0.49 0.80 0.20 0.33
carbon black (CB %)a
10.10 3.96 8.95 0.80 0.60 2.05 0.91
1.12 1.53
