Preparation and Characterization of Gel-like ... - ACS Publications

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Preparation and Characterization of Gel-like Dispersions Based on Cellulosic Pulps and Castor Oil for Lubricant Applications N. Nu~nez,† J. E. Martín-Alfonso,† M. E. Eugenio,‡ C. Valencia,† M. J. Díaz,† and J. M. Franco†,* †

Departamento de Ingeniería Química, Facultad de Ciencias Experimentales, Universidad de Huelva, Campus de “El Carmen”, 21071 Huelva, Spain ‡ INIA-CIFOR, Ctra de la Coru~ na, km 7.5. 28040, Madrid, Spain ABSTRACT: In this work, the use of different cellulosic pulp samples to prepare new formulations potentially applicable as biodegradable lubricating greases is explored. Cellulosic pulps from different raw materials and submitted to different pulping process and/or several acidic treatments were characterized and used as biothickener agents to formulate gel-like dispersions. Cellulose pulp samples mainly differed in polymerization degree (160893 cm3/g) and lignin (0.327.1% w/w) and R-cellulose (46.085.1% w/w) contents. Rheological measurements and some standard mechanical tests usually performed with lubricants were carried out. The rheological response of cellulosic pulp-based gel-like dispersions is mainly a consequence of the balance between the cellulose polymerization degree and lignin and R-cellulose contents, independently on the origin of cellulose samples and pulping treatment. An empirical correlation between a power function of these variables and the plateau modulus has been proposed to predict the rheological behavior of resulting formulations. However, cellulosic pulp-based dispersions studied generally present either poor mechanical stability or low consistency indexes than expected for lubricating greases.

1. INTRODUCTION From a general perspective, there is a marked tendency to increase the use of eco-friendly consumer products, as a result of government regulations, or due to increasing public concern for a pollution-free environment.1 Environment concerns are rapidly gaining in importance worldwide. Among many political and social pressures on governmental departments and organizations around the world, the introduction of the ecolabeling has had significant impact. The EU ecolabel scheme establishes criteria for groups of products and services in order to meet high environmental and performance standards. A relatively new group of products created in this scheme is that regarding lubricants.2 Besides this, the lubricant industry in the European Community must fit the REACH regulation3 dealing with the registration, evaluation, authorization, and restriction of chemical substances. Concerning the impact that lubricants exert on the environment, every year millions of tones of engine, industrial, and hydraulic oils are poured in the environment. Even small amounts of these products can inhibit the growth of trees and can be toxic to the aquatic life.4 For these reason, throughout the world today, the development of biodegradable materials with controlled properties is a great research challenge to the community of material scientists and engineers.5 Lubricating greases basically consist of a thickening agent, generally metal soaps or urea derivatives, dispersed in a lubricating liquid, mineral, or synthetic oil, forming a colloidal suspension,6,7 which presents some additional advantages in comparison to conventional lubricating fluids, mainly due to their characteristic rheological behavior.8 Thus, for instance, greases are the preferred lubricant in some specific applications like hard-to-reach places or high-speed bearings where leakage and sealing are particular concerns,7 with important functional properties related to their ability to flow under external forces, show optimum mechanical stability under shearing, dampen viscosity changes due to temperature and pressure r 2011 American Chemical Society

variations, seal out contaminants including water, and decrease dripping and spattering, etc.9 As the main components of lubricating greases are considered nonbiodegradable materials, there is a general tendency to promote the replacement of these by renewable resources. Up to now, main attempts to develop biogrease formulations have been limited to the replacement of mineral oils by vegetable ones.9,10 Among the vegetable oils, the castor oil is being occasionally used for lubricant purposes due to its high viscosity and good performance characteristics at low temperatures.11 However, the research of new competitive and efficient biodegradable thickeners, as substitutes for metal soaps or polyureas is much less explored and presumably a more complicated task. Thus, a biothickener should convey to biogrease formulation similar or improved rheological characteristics, and thermal and mechanical stabilities to those provided by traditional metallic soaps. Biopolymers are promising candidates to be used in those applications where biodegradability and/or the derivation of natural resources give added value. In previous studies,12 the possibility of using oleogels based on sorbitan monostearate and castor oil for lubricating purposes was explored. However, in spite of the promising rheological characteristics found and some benefits as a biodegradable alternative to greases, at least for specific applications, some limitations related to the maximum working temperature and their rather poor mechanical stability are apparent. On the other hand, gel-like dispersions based on castor oil and purified R-cellulose derivatives were proposed as potential biodegradable lubricating greases.13,14 Cellulosic derivatives present some advantages as lubricating grease thickeners. They are Received: December 22, 2010 Accepted: March 15, 2011 Revised: March 10, 2011 Published: March 23, 2011 5618

dx.doi.org/10.1021/ie1025584 | Ind. Eng. Chem. Res. 2011, 50, 5618–5627

Industrial & Engineering Chemistry Research biodegradable biopolymers obtained from the most abundant natural polymer5 and, on the other hand, they can provide suitable rheological properties to these formulations.15 In particular, the use of ethyl cellulose combined with other cellulose compounds, such as methylcellulose or R-cellulose, yields gel-like dispersions with acceptable thermal, rheological, and mechanical properties. In this sense, industrial cellulosic pulps appear as another interesting biodegradable alternative to replace the traditional thickener agents in lubricating greases. Some preliminary studies have been recently reported regarding the methylation of Kraft cellulose pulp to be used as thickener agent in such lubricant products.16 Owing to their abundance, low cost, and well-known processability, cellulosic pulps are currently attracting the attention of researchers and technologists in several fields to be used in a great variety of applications.17 In this work, the possibility of using different cellulosic pulp samples to prepare new formulations potentially applicable as biodegradable lubricating greases is explored. In particular, cellulosic pulps from different raw materials and submitted to different pulping/bleaching processes and/or several acidic treatments were characterized and tested as biothickener agents to formulate gel-like dispersions. These cellulose pulp samples mainly differ in polymerization degree and Rcellulose and lignin contents. The influence that cellulosic pulp characteristics exert on the rheological properties and mechanical stability of the resulting gel-like dispersions in castor oil was evaluated.

2. MATERIALS AND METHODS 2.1. Materials. Castor oil (211 cSt at 40 C, Guinama, Spain) was selected as biodegradable lubricating oil. Ethyl cellulose (Mn: 66000 g/mol; 49% ethoxy content) from Sigma-Aldrich was used as gelling agent to modify the castor oil rheological properties, as previously described.14 Different industrial cellulosic pulps were used to prepare gel-like dispersions in the castor oil medium. Commercial grade Kraft cellulose pulp from Eucalyptus globulus (sample EucalKraft) was kindly supplied by ENCE, S.A. (Huelva factory, Spain) together with other two cellulose pulp samples obtained at different intermediate pulping process stages of the factory, that is, outflow of digesters (sample Eucal-Kraft-Process1) and first process stage of bleaching (sample Eucal-Kraft-Process2), respectively. Apart from the industrial pulps supplied by ENCE, other pulps produced in the laboratory such as Kraft and mechanical pulps from Pinus radiata; mechanical and semimechanical pulps from Eucalyptus globulus, and a recycled pulp sample were also used in this work. Solvents and other chemicals employed were of reagent grade and used as received from Sigma-Aldrich. 2.2. Pulps Production. Pinus radiata wood was submitted to a Kraft cooking done in the laboratory (sample Pine-Kraft) by adding wood chips in a 26 L digester connected to a blowing tank through a pneumatic valve. The cooking liquor was indirectly heated with steam in an external tubular heat exchanger and was continuously fed back into the process. The cooking temperature was controlled by a computer program designed in our laboratories to regulate the admission of steam in the heater exchanger. In addition, this control system allows us to know the H factor (a control parameter of the pulping process which includes time and temperature in a single variable) at any time during the cooking process. The operational conditions were 20% active alkali, 25% sulfidity, liquor/wood ratio 4, cooking temperature 170 C, 40 min interval to cooking temperature, and 60 min at cooking temperature (corresponding to an H

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factor of 940). Then, cooked chips were washed, disintegrated, and screened to obtain the final pulp. Semimechanical cooking (sample Eucal-Semimech) was carried out using a 12 in. Sprout Waldron refiner and performed in the following steps. First, the raw material was immersed in a 0.15 M NaOH solution for 2 h. Afterward, the material was defibrated using a space of 0.5 mm between refiner discs at 70 C. The space between discs allowed us to obtain a pulp without high energy consumption. Finally, defibrated pulps were passed through a screen of 0.25 mm. Mechanical pulps (samples Eucal-Mech and Pine-Mech) were obtained in a similar way by avoiding the pretreatment with NaOH. Recycled pulp was obtained from recycled paper in a 50 L laboratory pulper, powered with a 2 CV engine and fitted with a helical rotor for high consistency purposes. Operational conditions were 10 min, 50 C, and 15% consistency. Finally, the commercial grade Kraft cellulose pulp from the ENCE factory was also submitted to subsequent different acid treatments thus altering their structure in order to widen their degree of polymerization maintaining the same composition. The acid treatments were carried out using HCl 13 M, at 7085 C and modifying the reaction time. HCl was selected because, under similar conditions, it produces lower hemicellulose hydrolyzation to xylose and other sugars, and subsequent breaking to furfural, than other acids like H2SO4.18 2.3. Cellulose Pulps Characterization. Morphological characterization of different cellulosic pulps was carried out by means of atomic force microscopy (AFM), with a Multimode apparatus connected to a Nanoscope-IV scanning probe microscope controller (digital Instruments, Veeco Metrology Group Inc., USA). All images were acquired in the tapping mode, using Veeco Nanoprobe tips. In addition to this, an optical microscope (Vanox AH3 Olympus connected to a digital camera Color View III) was used in order to quantify more precisely the different types of fibers in the recycled pulp. In this pulp sample, 90% of basic weight corresponded to hardwood bleached chemical pulp. Cellulosic pulps were characterized using standard methods to determine R-cellulose content (Tappi T-203), hemicellulose content (indirectly determined by analyzing the holocellulose content using the Wise method,19 lignin content (Tappi T-222), and intrinsic viscosity (Tappi T-230). The intrinsic viscosity, [η], provides an estimation of the average degree of polymerization (DP) of the cellulose fiber. The degree of polymerization of fibers was estimated using the following equations.20 DP ¼

½η for DP < 950 0:42

DP0:76 ¼

½η for DP > 950 2:28

ð1Þ

2.4. Manufacture of Gel-like Dispersions. The processing of gel-like dispersions was performed in an open vessel (400 g), using anchor impeller geometry to disperse the cellulosic pulps in the castor oil medium. In the first step, the vessel was filled with the corresponding amounts of oil and ethyl cellulose. A constant rotational speed (60 rpm) was then applied to the mixture, while this was heated to 150 C. Once the ethyl cellulose was completely dissolved, the mixture was cooled down to room temperature by natural convection, and cellulosic pulp was then slowly added and blended at the same rotational speed for other 30 min. Finally, a homogenization treatment (rotational speed, 5619

dx.doi.org/10.1021/ie1025584 |Ind. Eng. Chem. Res. 2011, 50, 5618–5627

Industrial & Engineering Chemistry Research

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Table 1. Composition and Intrinsic Viscosity of Cellulosic Pulp Samples Used in the Formulation of Biodegradable Lubricating Greases R-cellulosea (% w/w)

hemicelluloseb (% w/w)

Eucal-Kraft

85.1 ( 1.5

13.1 ( 1.4

0.3 ( 0.1

688 ( 14

Pine-Kraft Eucal-Mech

80.5 ( 2.7 47.2 ( 1.4

13.8 ( 1.6 36.5 ( 1.1

4.5 ( 0.1 18.6 ( 0.4

819 ( 16 584 ( 12

pulp/processing

a

ligninc (% w/w)

intrinsic viscosityd (cm3/g)

Pine-Mech

46.0 ( 3.4

22.4 ( 3.7

27.1 ( 0.2

578 ( 15

Eucal-Semimech

49.7 ( 0.7

30.2 ( 0.2

18.4 ( 0.2

770 ( 11

Recycl-Sulfate

76.2 ( 0.3

13.6 ( 0.6

8.6 ( 0.1

574 ( 9

Eucal-Kraft-Process1

73.8 ( 0.3

20.4 ( 0.8

1.4 ( 0.2

893 ( 11


79.5 ( 1.7

17.0 ( 1.5

1.2 ( 0.1

772 ( 12

Tappi T-203-05-61. b Wise et al., (1946). c Tappi T-222. d Tappi T-230.

8800 rpm), using a rotor-stator turbine (Ultra Turrax T-50, Ika, Germany), was applied at room temperature, achieving gel-like suspensions of cellulosic pulp in the castor oil medium modified with the ethyl cellulose. Ethyl cellulose and cellulose pulp concentrations were fixed at 3% and 4% w/w, respectively. 2.5. Rheological Tests. Rheological characterization of formulations studied was carried out in both controlled-stress (Rheoscope, ThermoHaake, Germany) and controlled-strain (ARES, Rheometric Scientific, UK) rheometers, using plateplate geometries (20 mm diameters, 1 mm gap). Smallamplitude oscillatory shear (SAOS) tests were performed inside the linear viscoelastic region, in a frequency range of 0.03100 rad/s and temperatures between 25 and 125 C. Stress sweep tests were previously performed to determine the linear viscoelastic regime (stress values between 1.5 and 8 Pa, depending on the consistency of the sample). At least two replicates of each test were carried out on fresh samples. 2.6. Thermogravimetric Analysis (TGA). Thermogravimetric measurements were performed with a Q-50 TA Instrument under a nitrogen gas flow of 100 mL/min. Approximately 10 mg of sample was placed on a Pt pan and heated from 30 to 600 C at a heating rate of 10 C/min. 2.7. Penetration and Mechanical Stability Tests. As usually tested in traditional lubricating greases, both unworked and worked penetration indexes were determined according to the ASTM D 1403 standard, by using a Seta Universal penetrometer, model 17000-2, with one-quarter cone geometry (Stanhope-Seta, UK). The one-quarter scale penetration values were converted into equivalent full-scale cone penetration values, according to the ASTM D 127 standard. Classical consistency NLGI grade was established according to these penetration values.7 Samples were worked during 2 h in a Roll Stability Tester, model 19400-3 (Stanhope-Seta, UK), according to the ASTM D 1831 standard, and penetration measurements were performed, once again, immediately after this rolling test. The mechanical stability of gel-like dispersions studied was then calculated as the difference between worked and unworked penetration values. 2.8. Leakage Tendencies in Wheel Bearings. Leakage tendencies in wheel bearings were determined in a Petrotest equipment, model 17-0450 (Germany), according to the ASTM D 1263 standard. This test method covers the evaluation of the leakage tendencies of lubricating greases when they are tested under specific simulated wheel bearing conditions. The sample is distributed in a modified front-wheel hub and spindle assembly. The hub is rotated at a speed of 660 ( 30 rpm for 6 h ( 5 min, at a

Table 2. Intrinsic Viscosity of Kraft Cellulosic Pulps Submitted to Different Acidic Treatments acidic treatment

pulp

a

time (h)

HCl

intrinsic

concentration (M)

viscositya(cm3/g)

Eucal-Kraft

0

0

688 ( 14

Eucal-Kraft-ac1

0.5

1

650 ( 12

Eucal-Kraft-ac2

1.5

1

535 ( 10

Eucal-Kraft-ac3 Eucal-Kraft-ac4

2 3

1 1

428 ( 9 333 ( 10

Eucal-Kraft-ac5

4

1

213 ( 7

Eucal-Kraft-ac6

1.5

2

203 ( 8

Eucal-Kraft-ac7

2.5

3

160 ( 5

Tappi T-230.

spindle temperature. In this case, temperature was raised and then maintained at 105 ( 1 C or 50 ( 1 C. Leakage of the sample was determined by weight at the end of the test.

3. RESULTS AND DISCUSSION 3.1. Characterization of Cellulosic Pulps. Table 1 shows the R-cellulose, hemicelluloses, and lignin contents as well as the intrinsic viscosity of pulp samples studied. As can be expected, the mechanical and semimechanical pulps present lower R-cellulose and higher hemicellulose and lignin contents than chemical pulps. Mechanical pulping does not use chemicals to eliminate lignin and hemicellulose. On the contrary, in the chemical pulping, delignification is carried out with the help of chemical reagents. Therefore, lignin and hemicellulose are partially eliminated. This can be clearly observed in Table 1 where low lignin and high Rcellulose contents are shown for Kraft pulps. In general, once bleached, these pulps are characterized by their low lignin content. The chemical composition obtained for the recycled pulp is quite similar to that found for eucalyptus chemical pulps, as a consequence of the similar composition of the recycled original material used (hardwood Kraft chemical pulp), although significantly higher lignin content was found. In addition to this, higher intrinsic viscosity values were obtained for Kraft pulps compared to those of the mechanical or semimechanical pulps, probably due to the removal of the short-chain celluloses in the chemical extractive steps, which led to higher average molecular weight of the obtained cellulose pulp.21 Among the chemical pulps, the subsequent lignin and 5620



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Figure 1. AFM micrographs (window size: 2 μm) of different cellulosic pulp samples: (a) Eucal-Kraft; (b) Pine-Kraft; (c) Recycl-Sulfate; (d) EucalMech; (e) Pine-Mech; (f) Eucal-Semimech.

Figure 2. Evolution of the storage and loss moduli (a) and the loss tangent (b) with frequency for gel-like dispersions prepared with different cellulosic pulp samples (G0 full symbols, G00 open symbols).

hemicellulose extractions in the pulping process does not significantly modify the viscosity values. Table 2 also listed the viscosity values for commercial eucalyptus Kraft pulp submitted to different acid treatments. The chemical composition of all these samples is essentially the same as that shown in Table 1 for the EucalKraft sample. The effect of acid treatment on pulp viscosity indicated that the higher-intensity acid treatment (higher HCl concentration and longer time) resulted in a progressive decrease in the pulp viscosity (from 688 to 160 cm3/g). This wide viscosity range indicates a dramatic degradation of cellulose polymeric chains. As previously reported,22 acid degradation occurs at weak points in the fiber cell wall. Although the viscosity data are insufficient for evaluating the different cellulose depolymerization reactions, the viscosity changes indicate that there

are differences in the homogeneity and selectivity of these degradation reactions and, therefore, different rheological properties of the resulting gel-like systems are expected. Figure 1 shows the different microstructures of different original cellulosic pulps obtained by using the AFM technique. As can be deduced from these micrographs, the morphology of cellulosic pulp is highly dependent on both the origin of the cellulosic material and the pulping treatment. In general, Pinus radiata pulps present more homogeneous microstructures with longer fibers and lower degree of ramifications than Eucalyptus globulus pulps. In addition to this, the application of mechanical pulping treatments generally yields fibers with higher length and diameters, especially in the case of mechanical Eucalyptus globulus pulp (see Figure 1d). As expected, an intermediate morphology was found for the Eucalyptus globulus pulp submitted to a semimechanical treatment although more similar to that found for the Kraft sample. Finally, the recycled pulp presents a rather heterogeneous morphology and relatively short fiber length in concordance with its composition. 3.2. Rheological Properties of Gel-like Dispersions. Figure 2 shows the evolution of the storage, G0 , and loss, G00 , moduli with frequency, at 25 C, within the linear viscoelastic range, for gel-like dispersions formulated using cellulosic pulps from different raw materials and/or submitted to different treatments as thickener agents. In all cases, ethyl cellulose was also added at 3% w/w as rheology modifier of castor oil. As previously reported,14 the main role of ethyl cellulose is to inhibit oil phase separation (oil bleeding) by forming a soft gel with the castor oil. In this sense, ethylcellulose impart long-term physical stability to these gel-like dispersions as a consequence of significantly increasing castor oil viscosity and also conferring some viscoelastic characteristics to the continuous medium. In general, G0 is always higher than G00 in the whole frequency range studied and the plateau region of the mechanical spectrum is always noticed, as extensively described elsewhere for gel-like disperse systems23,24 and, particularly, for standard lubricating greases.2529 Moreover, these mechanical spectra are qualitatively 5621



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Figure 3. Evolution of the storage and loss moduli (a) and the loss tangent (b) with frequency for gel-like dispersions prepared with different cellulosic pulp samples (G0 full symbols, G00 open symbols).

Figure 4. Evolution of the storage and loss moduli (a) and the loss tangent (b) with frequency for gel-like dispersions prepared with Eucalyptus globulus Kraft pulps submitted to different acidic treatments (G0 full symbols, G00 open symbols).

similar to those shown by conventional lithium lubricating greases, with G0 values typically ranged from 104 to 105 Pa, at 2575 C, around 1 order of magnitude higher than G00 values, depending on composition and processing conditions.30,31 As can be observed in Figure 2a, the mechanical spectra of oleogels containing Eucalyptus globulus pulps taken from the different stages of the Kraft processing plant are very similar with slight differences in the values of the SAOS functions. Thus, for instance, gel-like dispersions formulated using the final Eucalyptus globulus pulp or the sample taken from the outflow of digesters in the Kraft process (Eucal-Kraft-Process1), display comparable values of these linear viscoelastic functions, in spite of the differences found in lignin content and polymerization degree (Table 1). However, pulp taken from the first bleaching stage (Eucal-Kraft-Process2), with more similar polymerization degree but significantly higher lignin content than the final Kraft cellulosic pulp, provides a gel-like dispersion with lower values of the SAOS functions. On the other hand, this dispersion displays an almost identical mechanical spectrum as that shown by the Pinus radiata Kraft pulp-based oleogel, with slightly higher values of both lignin content and polymerization degree (Table 1). Finally, the formulation containing cellulosic pulp obtained from a recycled material shows the lower values of the SAOS functions probably due not only to the high lignin content and low degree of polymerization but also to the degradation degree of the cellulosic material, which yields a different and more heterogeneous fiber microstructure (Figure 1). However, the relative elastic characteristics of these gel-like dispersions are not significantly affected, as can be observed in Figure 2b where the loss tangent (tan δ = G00 /G0 ) is plotted versus frequency. Figure 3a compares the functional capacity of Eucalyptus globulus and Pinus radiata Kraft pulps to achieve gel-like dispersions in castor oil with those shown by cellulosic pulps obtained from mechanical or semimechanical processing. As can be observed, the application of mechanical pulping on the original cellulosic materials significantly reduces the strength of the resulting oleogel formulations providing values of the SAOS functions around one decade lower than those of the Kraft pulpbased gel-like dispersions, also associated to an important reduction of the relative elastic behavior as can be observed in Figure 3b. As expected, the application of a semimechanical

treatment with soda produces a cellulosic pulp with intermediate gelling characteristics to those previously discussed thus resulting a gel-like dispersion with intermediate values of the SAOS functions, otherwise very similar to those found in traditional lithium lubricating greases.2527 As can be observed in Table 1, apart from different polymerization degree, all these cellulosic pulps differ in the composition. This fact suggests that the higher the lignin and the lower the R-cellulose contents are, the lower are the obtained values of the oleogel SAOS functions. The influence of polymerization degree on the viscoelastic properties of cellulosic pulp-based dispersions can be isolated by applying different acidic treatments on a given cellulosic pulp. In this sense, as it has been previously mentioned, Eucalyptus globulus Kraft pulp was subjected to different acid treatments breaking up cellulose and hemicelluloses thus altering the structure of the original pulp and mainly the polymerization degree (Table 2). Figure 4 displays the evolution of SAOS moduli with frequency, within the linear viscoelasticity range, at 25 C, for selected dispersions containing Eucalyptus globulus Kraft pulp. Once again, in most cases, the values of the storage modulus, G0 , are higher than those found for the loss modulus, G00 , in the whole frequency range studied, and the plateau region is generally noticed (Figure 4a). However, cellulosic pulps submitted to the most drastic acidic treatments yield weak gellike dispersions with a not well-developed plateau region. In fact, the slope of the G0 versus frequency plot progressively increases by decreasing the polymerization degree, associated to a significant decrease in the values of both SAOS functions (Figure 4a) and relative elastic characteristics (Figure 4b). The application of a relatively mild acid treatment only produces a minor modification of the rheological response respecting the nontreated cellulosic pulp-based oleogel sample. For instance, a slight modification is obtained by using cellulosic pulp treated with 1 M chlorhydric acid, at temperatures of 7080 C, and reaction time of 2 h. On the contrary, a significant modification was achieved by reducing the polymerization degree three times that of the original Kraft pulp (sample Eucal-Kraft-ac5). A more drastic acid treatment with 3 M chlorhydric acid and reaction time of 2.5 h on the cellulosic pulp yields a completely different rheological behavior close to a critical gel where G00 is, in this case, slightly higher than G0 following a power-law evolution 5622



Figure 5. Evolution of the plateau modulus of gel-like dispersions with the polymerization degree of Eucalyptus globulus Kraft pulps submitted to different acidic treatments, at different temperatures.

Figure 6. Correlation between the plateau modulus of gel-like dispersions and a power series of cellulose polymerization degree and Rcellulose and lignin weight fractions (full symbols, cellulosic pulps from different raw materials and submitted to different cooking or bleaching processes; open symbols, Eucalyptus globulus Kraft pulps submitted to different acidic treatments).

with frequency. Evidently, this rheological behavior diverges from that required for a lubricating grease. Figure 5 illustrates the effect of polymerization degree on the linear viscoelastic response, at different temperatures, of gel-like dispersions prepared with cellulosic pulps submitted to different acidic treatments. In this Figure, the plateau modulus, G0N, a characteristic parameter of the plateau region in the mechanical spectrum is plotted versus DP, estimated according to eq 1. The plateau modulus, G0N, defined for the polymers as the extrapolation of the contribution of the entanglements to G0 at high frequencies,32 may be considered as a measure of the aggregation number among the dispersed structural units and, consequently, it is related to the strength of the microstructural network. Different methods for the determination of the plateau modulus have been used.33 In this work, G0N was straightforwardly estimated as the G0 value at a frequency for which the loss tangent is minimum.34,35 This method was applied for further comparison even though the plateau region of the mechanical spectrum is not

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Figure 7. Evolution of the storage and loss moduli with frequency, at different temperatures, for a selected gel-like dispersion prepared with the Eucal-semimech pulp sample (G0 full symbols, G00 open symbols).

clearly detected, that is, with cellulose pulp samples submitted to drastic acidic treatments (Eucal-Kraft-ac56). As can be observed, G0N increases almost linearly with the polymerization degree up to a critical value and then remains almost constant. According to all these aforementioned results, it can be concluded that the rheological response of these cellulosic pulpbased gel-like dispersions studied is mainly a consequence of the balance between DP and lignin and R-cellulose contents. Thus, for instance, if the cellulosic material polymerization degree is similar, the higher the lignin content is, the lower are the values of the viscoelasticity functions. Aiming to quantify these opposite effects, the plateau modulus of the gel-like dispersions was plotted versus a product of powers of the polymerization degree and Rcellulose and lignin weight fractions (Figure 6), where R-cellulose and lignin contents have the same but reciprocal weight in this function but 1/4 lower than DP: G0N ¼ c1 ½DP 3 ØR celluloseø

1=4

3 ½lignin

1=4

þ c2

ð2Þ

where c1 and c2 are fitting parameters. The values of power-law exponents were deduced from an optimum linearization of the evolution of G0N with this power series, that is, the best linear regression (R2 = 0.965). Thus, as can be observed in Figure 6, G0N increases linearly with this power function in the experimental range of DP and composition studied, independently on the kind, nature, or processing of the cellulosic materials employed as thickener agents. This is an important finding that allows a prediction of the rheological properties of dispersions formulated with a given cellulose pulp. On the other hand, Eucalyptus globulus Kraft pulp-based dispersions present a certain degree of oil phase separation (oil bleeding), especially when using cellulosic pulps with lower lignin content, which clearly represents an important disadvantage for use as lubricating greases. However, the use of either Pinus radiata pulp, in general, or Eucalyptus globulus pulp obtained from mechanical pulping, both of them containing a higher amount of lignin, significantly dampens oil phase separation. Figure 7 shows the evolution of SAOS functions with frequency, as a function of temperature, for a selected formulation 5623



Figure 8. Evolution of the plateau modulus with temperature and fitting to eq 3, for a selected cellulosic pulp-based gel-like dispersions.

Table 3. Activation Energy Values Deduced from the Fitting to eq 3 for Different Cellulose Pulp-Based Gel Like Dispersions Studied pulp/processing Eucal-Kraft Pine-Kraft

Ea (J/mol) 8118 1463

Eucal-Mech

13639

Pine-Mech Eucal-Semimech

6118 10236

Recycl-Sulfate

5529

Eucal-Kraft-ac1

9987

Eucal-Kraft-ac3

14248

Eucal-Kraft-ac5

20174

(Eucal-semimech-based dispersion). As can be expected, the values of SAOS functions decrease with temperature in the whole frequency range studied, although a well developed plateau region is always noticed. On the other hand, the frequency dependence of both moduli is not qualitatively influenced by temperature, a fact that is completely opposite to the behavior reported for standard lithium lubricating greases.36 Moreover, the loss tangent is almost unaffected, which indicates that the relative elastic characteristics are not influenced by temperature either, as identically found in oleogels processed with purified Rcellulose or methyl cellulose.14 However, at temperatures higher than 125 C, a significant oil bleeding is generally observed, which limits the thermo-rheological characterization. As previously reported for standard lithium greases,31 the plateau modulus can be used to quantify the influence of temperature by using an Arrhenius-type equation (R2 > 0.86): Ea 1 G0N ¼ A exp ð3Þ R 3T where Ea is a parameter which evaluates the thermal dependence, similar to the activation energy (J/mol), R is the gas constant (8.314 J/mol K), T is the absolute temperature (K), and A is the pre-exponential factor (Pa). Equation 3 fits fairly well the experimental plateau modulus values, in the whole temperature range studied, as can be observed in Figure 8 for selected gel-like dispersions differing in the pulping treatment assayed. Table 3

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Figure 9. TGA thermograms for (a) selected gel-like dispersions and (b) the corresponding cellulosic pulps used in their preparation.

collects the Ea values, obtained from eq 3, for different cellulosic pulp-based gel-like dispersions studied. As can be observed in Table 3, the application of mechanical or semimechanical pulping treatments on the cellulosic materials used as thickener agents produces dispersions with higher thermo-rheological susceptibility, that is, higher Ea values. However, the recycled pulp yields dispersions with low G0N-temperature dependence. Besides this, the application of acidic treatments on cellulosic pulps produces a higher thermo-rheological susceptibility on the corresponding oleogels, which becomes more important as the more drastic the acidic treatment is. Finally, Ea values obtained for all cellulosic pulp-based gel-like dispersions are higher than those obtained for standard lithium lubricating greases in the lowtemperature range (000

500

15

Table 6. Leakage Values, at 50C and 105C, for Different Cellulose Pulp-Based Gel Like Dispersions Studied leakage tendency

leakage tendency

(at 50 C) (%)

(at 105 C) (%)

Eucal-Kraft

4.0

23.4

Pine-Kraft Eucal-Mech

9.8

27.3 40.6

pulp/processing

Pine-Mech

42.8

Eucal-Semimech

37.8

Recycl-Sulfate

24.9


23.0


21.5

Eucal-Kraft-ac1 Eucal-Kraft-ac3 Eucal-Kraft-ac4 Eucal-Kraft-ac5

23.5 8.1

25.0 47.9 69.6

contributes in a low weight loss percentage. Very similar characteristic TGA parameters were obtained in gel-like dispersions of commercial R-cellulose or methyl cellulose in castor oil.13 Besides this, these biodegradable formulations studied can resist much higher temperatures, without losing a significant amount of mass, than paraffinic or naphthenic oil-based lubricating greases.27 Thus, the main factor affecting TGA response is the volatility of the base oil used in these formulations. 3.4. Standard Mechanical Tests. One of the technical parameters more often used by lubricating grease manufacturers is the mechanical stability determined from penetration tests before and after working the sample. Mechanically stable

lubricating greases display penetration increments (before and after working the sample) close to zero. In this study, cellulosic pulp-based oleogel samples were worked in a roll stability tester. Penetration values of unworked samples were also converted to NLGI grades, as traditionally carried out to classify greases according to its consistency degree (between 000 and 6). The most commonly used greases are those with NLGI grade 2. Softer grades, especially 0 and 1, are sometimes used for improved pumpability or low-temperature applications, while higher consistency indexes are used for certain high-speed bearings.7 Table 5 shows worked and unworked penetration values, as well as penetration variation (mechanical stability), for all oleogels studied. As can be observed in Table 5, oleogel samples containing Kraft pulp, or cellulosic pulps coming from different stages of the industrial Kraft process, show similar unworked penetration values (NLGI grades 12), slightly lower in the case of the Pinus radiata Kraft pulp-based oleogel (NLGI grade 23). However, in this last case, the penetration increment is significantly higher, indicating a poor mechanical stability. Much lower consistency shows oleogels manufactured with pulps submitted to mechanical or semimechanical treatment (NLGI grades 000), with higher lignin content, also yielding penetration increments generally higher. NLGI grade 00 samples do not show excessively low mechanical stability as a consequence of the very low initial penetration values. On the other hand, cellulosic pulps submitted to acid treatments forms gel-like dispersions with lower consistency, especially those submitted to more drastic conditions (Table 5), as expected attending to the lower cellulose polymerization degree. The mechanical stability of oleogels also decreases with the strength of the acid treatment, excepting once again for nonstructured samples 5625


Industrial & Engineering Chemistry Research (NLGI grade 00 or 000) which exhibit very low unworked penetration values. Moreover, selected oleogel samples were submitted to the standard leakage tendency test, at 105 C. Table 6 collects these results. In general, Kraft cellulosic pulp-based dispersions shows leakage tendencies of around 25%, significantly higher than that expected for lubricating greases. These values increase when using cellulosic pulp submitted to either a mechanical pulping or acid treatments. As Sanchez et al.13 previously discussed, this high level of leakage is related to an important softening of the ethyl cellulose/castor oil binary systems detected at around 60 C. Above this temperature, both the viscosity and relative elastic properties of the oily phase are significantly reduced and, therefore, the leakage is favored. Thus, a significant improvement in the leakage tendency was found when the same tests were performed at 50 C, as shown in Table 6 for selected formulations.

4. CONCLUSIONS In this work, the use of different cellulose pulps to obtain gellike dispersions in castor oil has been explored in order to find biodegradable alternatives to lubricating greases. The rheological behavior of these cellulosic pulp-based gel-like dispersions depends on the nature of lignocellulosic material (Pinus radiata and Eucalyptus globulus), the type of pulp cooking and the acidic treatment applied. The different raw materials employed and pulping treatments allow the acquisition of cellulose pulp samples with polymerization degree ranging from 893 to 160 cm3/g and different chemical composition (lignin and Rcellulose contents of 0.327.1% and 46.085.1%, respectively). The different rheological response of the resulting gel-like dispersions obtained by modifying these parameters covers the wide spectrum found in traditional lubricating greases of NLGI grades 003. In this sense, the rheological behavior was found to be mainly a consequence of the balance between the polymerization degree and lignin and R-cellulose weight fractions. An empirical linear correlation between the plateau modulus and a power function of these three variables has been proposed to predict the rheological response of the resulting gel-like dispersions, independently on the nature or processing of the cellulosic materials employed as thickener agent. In addition to this, oleogel formulations studied display much higher decomposition temperatures than standard lubricating greases, independently of the cellulose pulp employed. Gel-like dispersions containing eucalyptus cellulosic pulps coming from different stages of the industrial Kraft process show suitable consistency and mechanical stability to be used as lubricating greases. However, in general, cellulosic pulp-based dispersions in castor oil present either a poor mechanical stability or low consistency indexes, especially those prepared with cellulose pulps submitted to drastic acid treatments or processed using a mechanical cooking. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ34959219995. Fax: þ34959219983. E-mail: [email protected]. Address: Departamento de Ingeniería Química, Campus de “El Carmen”, Universidad de Huelva, 21071 Huelva, Spain.

’ ACKNOWLEDGMENT This work is part of a research project (CTQ2007-60463) sponsored by a MEC-FEDER program. N. Nu~ nez has received a

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Ph.D. Research Grant from the “Ministerio de Ciencia e Innovacion”. The authors gratefully acknowledge their financial support. Thanks are also given to ENCE S.A. (Huelva factory) for the supply of industrial cellulose pulp samples.

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