Influence of Eucalyptus globulus Kraft Pulping ... - ACS Publications

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Influence of Eucalyptus globulus Kraft Pulping Severity on the Rheological Properties of Gel-like Cellulose Pulp Dispersions in Castor Oil N. Núñez,† J. E. Martín-Alfonso,† M. E. Eugenio,‡ C. Valencia,†,§ M. J. Díaz,†,§ and J. M. Franco*,†,§ †

Departamento de Ingeniería Química, Campus de “El Carmen”, Universidad de Huelva, 21071 Huelva, Spain, Campus de Excelencia Internacional Agroalimentario, ceiA3 ‡ INIA-CIFOR, Ctra de la Coruña, km 7.5. 28040, Madrid, Spain § PRO2TECChemical Process and Product Technology Research Center, Universidad de Huelva, 21071 Huelva, Spain ABSTRACT: The development of some cellulose pulp gel-like dispersions in castor oil with application as biodegradable lubricants was explored in this work by analyzing the effect that Kraft cellulosic pulp cooking conditions exerts on the rheological properties and mechanical stability of these formulations. With this aim, Eucalyptus globulus cellulosic pulps were obtained by increasing the pulping time at the maximum temperature, yielding H factors ranging from 150 to 500. Gel-like dispersions of cellulosic pulps in castor oil were prepared and characterized from a rheological point of view. Small-amplitude oscillatory shear (SAOS) measurements and some standard mechanical tests, usually performed on commercial lubricating greases, were carried out in order to evaluate the lubricant performance of these dispersions. In general, the values of linear viscoelastic functions increase with the H factor. Cellulose pulps submitted to lower H factors provides gel-like dispersions with SAOS functions values more similar to those found in traditional lubricating greases. Results were explained attending to a balance between the cellulose polymerization degree and lignin and α-cellulose contents, all of them affected by the H factor applied. An empirical correlation between these variables and both the plateau modulus and the loss of consistency has been proposed to predict the rheological behavior and mechanical stability of gel-like cellulose pulp dispersions in castor oil.

1. INTRODUCTION It is well-known that the use of renewable resources for different industrial applications has remarkable importance in our society due to its positive effect on the environment. For these reasons, throughout the world today, the development of biodegradable materials, including cellulose-based products, with controlled properties is a subject of great research challenge to the community of material scientists and engineers.1 In this sense, there has been increasing demand for green products suitable to be used as lubricants2 in order to minimize the contamination damage that petroleum-based products cause in the environment. Lubricating greases basically consist of a thickening agent, generally metal soaps or urea derivatives, dispersed in a lubricating liquid, traditionally a mineral or synthetic oil, thus forming a colloidal suspension3,4 with characteristic performance properties that mainly depends on the rheological behavior.5 Taking into account that the majority components of lubricating greases are considered nonbiodegradable materials, there is a general tendency to promote the replacement of these by renewable resources. In particular, the achievement of environmentally friendly thickeners agents is one of the main targets in this type of lubricants. Cellulosic derivatives may present some advantages as lubricating grease thickeners since they are biodegradable biopolymers obtained from the most abundant natural polymer,1 and on the other hand, they can provide suitable rheological properties depending on several molecular properties.6 In a previous work,7 the use of cellulosic pulps from © 2012 American Chemical Society

different origin as biothickener agents to formulate gel-like dispersions in vegetable oils was explored. The rheological response of these gel-like dispersions was found to be mainly dependent on both α-cellulose and lignin contents and polymerization degree, also influenced by the type of pulping treatment and acidic treatments. As a continuation, a more systematic study on the influence of pulping process severity on the rheological characteristics of these cellulose pulp dispersions is presented in this work for standard and wellknown pulping process and raw material like the Kraft process and Eucalyptus globulus, respectively. The Kraft pulping process, introduced in 1879, is the dominant pulping process today and is likely to remain so in the near future.8 Thus, during Kraft pulping, the chemical components of wood undergo severe modifications.9 More than half of the total amount of hemicelluloses and nearly all the lignin are dissolved from the fibers, while the cellulose is partially degraded but not dissolved and its crystallinity degree increases.10−12 In the past 25 years many studies of the kinetics and transport behavior of the Kraft pulping process have been reported.9,13,14 With the data from these studies, Kraft pulping models of varying complexity have been developed for control and design purposes. The simplest of these models are those which assume that the pulping reaction rates are kinetically controlled. One of the earliest Received: Revised: Accepted: Published: 9777

April 18, 2012 June 27, 2012 June 28, 2012 June 28, 2012 dx.doi.org/10.1021/ie301014v | Ind. Eng. Chem. Res. 2012, 51, 9777−9782

Industrial & Engineering Chemistry Research

Article

kinetic models was developed by Vroom (1957),15 who used an Arrhenius type expression for the reaction rate-temperature dependence, namely the “H-factor”. The H factor, which combines the cooking temperature and cooking time into one variable, is the core of many control schemes used for Kraft pulping processes.8 Although this factor is widely used to control the cooking time commercial production, its application is limited to simple processes in which the change of cooking chemical concentrations are insignificant.9 Taking into account these considerations, the effect that Kraft cellulosic pulp cooking conditions, i.e. the H factor, exerts on the rheological properties and mechanical stability of some cellulose pulp gellike dispersions in castor oil, potentially applicable as biodegradable lubricating greases, is analyzed in this study.

DP =

DP0.76 =

H=

∫0

e

dt

for DP < 950

[η] for DP > 950 2.28

(2)

2.4. Preparation of Gel-like Dispersions. Gel-like dispersions were prepared in a mixing batch reactor (400 g), using an anchor impeller. In the first step, the vessel was filled with the corresponding amounts of oil and ethyl cellulose. This blend was heated up to 150 °C under stirring at 60 rpm. Once the ethyl cellulose was completely dissolved, the mixture was cooled down to room temperature and cellulosic pulp was then slowly added and stirred for 30 min. Finally, a homogenization treatment (rotational speed: 8800 rpm), using a rotor-stator turbine (Ultra Turrax T-50, Ika, Germany), was applied at room temperature. Ethyl cellulose and cellulose pulp concentrations were fixed at 3% and 4% w/w, respectively. 2.5. AFM Observations. Morphological characterization of 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. 2.6. Rheological Tests. Rheological characterization of cellulose pulp-based gel-like dispersions was carried out in a controlled-stress (Rheoscope, ThermoHaake, Germany) rheometer, using a plate−plate geometry (20 mm diameters, 1 mm gap). Small-amplitude oscillatory shear (SAOS) tests were performed inside the linear viscoelastic region, in a frequency range of 0.03−100 rad s−1 and temperatures comprised between 25 and 125 °C. At least two replicates of each test were carried out on fresh samples. 2.7. Penetration and Mechanical Stability Tests. Penetration indexes were determined according to the ASTM D 1403 standard on both unworked and worked Oleogel samples, by using a Seta Universal penetrometer, model 170002, 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 usually reported for conventional lubricating greases was calculated according to these penetration values.4 Cellulosebased gel-like dispersions 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 estimated 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), at 105 °C, according to the ASTM D 1263 standard. This test evaluates the leakage tendencies of lubricating greases by means of weight losses under specific simulated wheel bearing conditions.

2. MATERIALS AND METHODS 2.1. Materials. Castor oil (211 cSt at 40 °C, Guinama, Spain) was selected as a biodegradable lubricating oil. Ethyl cellulose (Mn 60000 g mol−1; 49% ethoxy content) from SigmaAldrich was used as gelling agent to modify the castor oil rheological properties, as previously described.16 Several eucalyptus cellulosic pulp samples differing in cooking time were used to prepare gel-like dispersions in the castor oil medium. A commercial grade bleached Kraft cellulose pulp from Eucalyptus globulus, kindly supplied by ENCE, S.A. (Huelva factory, Spain), having an intermediate Tappi viscosity value among those obtained for studied samples, was taken as reference. 2.2. Pulps Production. Kraft cooking was performed on Eucalyptus globulus chips in a 26 L batch reactor furnished with a system for recirculation and heating of the cooking liquor. This raw material was selected due to their abundance and low cost processability. The cooking temperature was controlled by a computer running specially developed software. Cooking conditions were the following: 1000 g of dry chips, 4 L/kg liquor to wood ratio, 16% active alkali, 20% sulfidity, 160 °C cooking temperature, and 40 min to maximum temperature. Additionally, a 15 min homogenization treatment was applied. Time at maximum temperature was varied to obtain an H factor ranging from 150 to 500 (a control parameter in the pulping process which includes time and temperature conditions). The H factor was calculated according to the following equation:15 ⎛ 16115 ⎞ t ⎜⎝43.181 − T (t ) ⎟⎠

[η] 0.42

(1)

in which T is the temperature (K) and t is the time (h). In that form, H factor depends on both time and temperature by combining both variables into a single variable thus representing the advance of the cooking process. After the cooking process was completed, the resulting pulps were washed and screened for further characterization. 2.3. Characterization Methods of Cellulosic Pulps. Cellulosic pulp samples were characterized using standard methods to determine α-cellulose content (Tappi T-203), hemicellulose content (Wise method),17 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 lignocellulosic fibers. The degree of polymerization was estimated according to the method proposed by Gericke (2009):18

3. RESULTS AND DISCUSSION 3.1. Characterization of Cellulosic Pulps. Table 1 shows the α-cellulose, hemicellulose, and lignin content as well as the intrinsic viscosity of the different cellulosic pulp samples studied as a function of the H factor. As expected, an increase in 9778

dx.doi.org/10.1021/ie301014v | Ind. Eng. Chem. Res. 2012, 51, 9777−9782


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Table 1. Composition and Intrinsic Viscosity of Cellulosic Pulps Samples Used in the Formulation of Biodegradable Lubricating Greases pulp/ processing

α-cellulosea (% w/w)

H 150 H 200 H 300 H 400 H 500 Ind. EucalKraft

77.6 77.7 77.7 83.1 83.7 85.1

a

± ± ± ± ± ±

0.14 0.15 0.16 0.17 0.12 0.13

hemicellulosesb (% w/w) 11.6 13.3 14.9 10.5 13.2 13.1

± ± ± ± ± ±

0.04 0.04 0.03 0.02 0.01 0.01

ligninc (% w/w) 3.7 3.0 1.8 1.1 0.7 0.3

± ± ± ± ± ±

0.18 0.25 0.34 0.30 0.28 0.28

intrinsic viscosityd (cm3 g−1) 442 633 848 749 705 688

± ± ± ± ± ±

18 25 34 30 28 27

Tappi T-203-05-61. bWise et al.17 cTappi T-222. dTappi T-230.

cooking time, i.e. H factor, produces an increase in α-cellulose content and a decrease in lignin content. Composition achieved with an H factor of 500 is very similar to that found in the industrial Kraft cellulose pulp sample taken as reference. Under Kraft pulping conditions, carbohydrates in the fibers are degraded. The most important reactions of these carbohydrates are endwise depolymerization and alkaline hydrolysis of wood polymer chains. The effect of the depolymerization reactions on the molar mass of cellulose is however small, compared to the effect of alkaline hydrolysis. Nevertheless, although the molar mass of cellulose is rapidly reduced, the effect on the yield of the pure alkaline chain scission is low.19 In that form, hemicelluloses are partially degraded during Kraft pulping, and the loss of hemicelluloses is responsible for most of the total yield loss observed during Kraft pulping.20 Therefore, hemicellulose is partially degraded and dissolved, extractives are removed to a large extent, and the lignin is extensively degraded and dissolved increasing cellulose concentration in the pulp obtained.21 In general, the viscosity is a function of cellulose fiber degradation in pulping processes (Tappi T-230). In this sense, temperature was the main driving force for damaging of cellulose pulp structure on the base of decreasing of related viscosity.22 It is interesting to observe that, on the contrary, intrinsic viscosity initially increases with the H factor, probably due to the removal of hemicelluloses and short-chain celluloses in the extractive step, and then decreases as a result of the more severe cooking treatment. The effect of the H factor on the morphology of cellulose fibers is illustrated in Figure 1. As can be clearly observed, the application of low H factors yields thicker and agglomerated fibers in contrast to high H factors, which produces a more homogeneous structure where thin and isolated fibers can be clearly distinguished. Fiber size obtained for an H factor 150 is better visualized in Figure 1b (10 μm window size). 3.2. Rheological Properties of Gel-like Dispersions. Figure 2 shows the evolution of SAOS functions with frequency, at 25 °C, within the linear viscoelastic range, for the different cellulose pulp-based gel-like dispersions studied as a function of the H factor applied in the pulping treatment. As can be noticed, G′ is always higher than G″ in the whole frequency range studied and the plateau region of the mechanical spectrum is always apparent, as extensively described elsewhere for solid-like disperse systems23,24 and, particularly, for standard lubricating greases,25,26 which otherwise corresponds with the definition given by Almdal et al. (1993)27 for solid-like gels. Moreover, the range of G′ and G″ values obtained using cellulose pulps submitted to different

Figure 1. AFM micrographs of cellulosic pulp samples submitted to different H factor treatments: (a) H = 150 (window size: 2 μm), (b) H = 150 (window size: 10 μm), (c) H = 300 (window size: 2 μm), and (d) H = 500 (window size: 2 μm).

Figure 2. Evolution of the storage and loss moduli (a) and the loss tangent (b) with frequency for gel-dispersions prepared with cellulosic pulp samples studied (G′ full symbols, G″ open symbols).

cooking times are similar to those shown by conventional lithium lubricating greases, with G′ values typically comprised between 104 and 105 Pa, at 25−75 °C, around 1 order of magnitude higher than G″ values, depending on composition and processing conditions.28,29 On the other hand, as can be observed in Figure 2a, the mechanical spectra of oleogels containing cellulose pulps submitted to lower cooking times (H-150; H-200) are almost identical. Something similar occurs with dispersions prepared with cellulose pulps submitted to intermediate cooking times (H-300; H-400), with only slight differences in the values of the SAOS functions. However, as a general tendency, the values of SAOS functions increase with the H factor. Thus, for instance, gel-like dispersion formulated using cellulose pulp submitted to H-500 shows the highest values of the SAOS functions, 9779



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comparable to those found in the Oleogel prepared with the industrial Eucalyptus globulus pulp. As can be observed in Table 1, all cellulosic pulp samples differ in both composition and degree of polymerization. Thus, rheological results just described indicate that the higher lignin and the lower αcellulose contents are, the lower values of SAOS functions are obtained for gel-like dispersions. In this case, cellulose pulps submitted to lower H factors provides gel-like dispersions with G′ and G″ values more similar to those found in commercial lubricating greases.26 On the contrary, the degree of polymerization does not seem to exert a direct influence on the rheological response of these oleogels. Besides this, 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 = G″/G′) is plotted versus frequency. This means that the H factor influences in the same extension the elastic and the viscous components of resulting gel-like cellulose dispersions in castor oil. According to these results, it must be deduced that the rheological response of these cellulosic pulp-based gel-like dispersions studied is a consequence of the combined influence of DP and lignin and α-cellulose contents. Aiming to quantify these combined effects, an empirical linear correlation between the plateau modulus of the gel-like dispersions, defined elsewhere,25 and a power function of DP and α-cellulose and lignin weight fractions was established (Figure 3), as previously

Figure 4. Evolution of the storage and loss moduli with frequency, at different temperatures, for a selected gel-like dispersion prepared with H-300 pulp sample. (G′ full symbols, G″ open symbols).

decrease with temperature, being almost identical above 80 °C. Very similar G″ values were also found at low frequencies. However, the minimum found in G″ is shifted to higher frequencies as temperature increases. A significant oil bleeding is generally observed at temperatures higher than 125 °C, which limits the thermorheological characterization. As previously reported,7 the plateau modulus can be used to quantify the influence of temperature by using an Arrheniustype equation: GN° = Ae Ea / RT

(3)

where Ea is a parameter which evaluates the thermal dependence, similar to the activation energy (J mol−1), R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature (K), and A is the pre-exponential factor (Pa). As can be observed in Figure 5 for selected gel-like dispersions, eq

Figure 3. Correlation between the plateau modulus of gel-like dispersions and a power series of cellulose polymerization degree and α-cellulose and lignin weight fractions.

proposed for gels prepared with cellulose pulps from different origin.7 As can be observed in Figure 3, in the range of compositions studied, the plateau modulus increases linearly with a power series of α-cellulose and lignin contents, with the same but reciprocal weight in this function, and DP with a higher weight. The values of power-law exponents were deduced from an optimum linearization of the evolution of GN° with this power series, i.e. the best linear regression (R2 = 0.851). Linear fitting deduced in the previous work with different cellulose pulps is also included in Figure 3 for the sake of comparison, indicating a similar linear trend of this rheological parameter with pulp composition, independently of the type cellulose pulp employed to prepare the gel-like dispersion. The influence of temperature on SAOS functions of cellulose pulp-based gel-like dispersions is illustrated in Figure 4 for a selected formulation. As can be seen, the values of G′ slightly

Figure 5. Evolution of the plateau modulus with temperature and fitting to eq 2, for a selected cellulosic pulp-based gel-like dispersion.

3 fits (R2 > 0.870) the experimental plateau modulus values fairly well, in the temperature range studied. Table 2 displays the Ea values, obtained from eq 3, for the cellulose pulp-based gel-like dispersions analyzed as a function of the H factor. As can be observed in Table 2, an increase in the H factor initially decreases the thermorheological susceptibility of resulting oleogels and then increases. Thus, the minimum Ea value was obtained for Oleogel prepared using cellulose pulps submitted 9780



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Table 2. Activation Energy Values Deduced from the Fitting to Equation 3 for Cellulose Pulp-Based Gel-like Dispersions pulp/processing

Ea (J mol−1)

H 150 H 200 H 300 H 400 H 500 Ind. Eucal-Kraft

11093 7344 2989 4932 5961 8118

to an H factor of 300, whereas pulps submitted to H factors treatments of 400−500 display a slightly higher thermal dependence. However, in this case, the thermal susceptibility of the Oleogel prepared with industrial Kraft cellulose pulp is much higher, similar to that found in gel-like dispersions of cellulose pulp samples submitted to the gentlest pulping treatments. Finally, Ea values obtained for gel-like dispersions prepared with cellulose pulps submitted to intermediate or high H factors are only slightly higher than those obtained for standard lithium lubricating greases in the low-temperature range ( 0.937). However, mechanical stability of these oloegels must be still improved to be comparable to those exhibited by commercial lubricating greases. Moreover, Oleogel samples were submitted to the standard leakage tendency test, at 105 °C (Table 3). In general, leakage tendency decreases by increasing the H factor in the pulping treatment, which suggests a relationship between this tendency and the rheological behavior of these oleogels. Formulations prepared with pulps submitted to higher cooking times showed moderate leakage tendencies of around 20−25% but still significantly higher than that expected for lubricating greases. However, in this case, as Sánchez et al.31 previously discussed, the high level of leakage must be related to an important softening of the ethyl cellulose/castor oil binary systems detected at around 60 °C. Above this temperature, the viscosity of the oily phase is significantly reduced and, therefore, the leakage is favored. Therefore, much lower leakage tendencies should be expected if this test was performed below 60 °C, as previously demonstrated.7

4. CONCLUSIONS The rheological response of cellulosic pulp-based gel-like dispersions studied is significantly affected by the cellulose pulping conditions applied. Cellulose pulps submitted to different H factors exhibit different polymerization degree (DP) and α-cellulose and lignin contents, yielding highly different structural morphology as shown in AFM observations.

Table 3. Penetration and Leakage Values and Mechanical Stability for Cellulose Pulp-Based Gel-like Dispersions pulp/processing H 150 H 200 H 300 H 400 H 500 Ind. Eucal-Kraft

unworked penetration (dmm) NLGI grade 343 331 326 320 290 313

0−1 1 1 1 2 1

worked penetration (dmm) penetration variation (dmm) 399 377 362 347 324 343 9781

56 46 36 31 34 30

leakage tendency (at 105 °C) (%) 33.2 30.4 29.1 27.7 21.6 23.4



Article

(12) Hult, E. L.; Larson, P. T.; Iversen, T. A comparative CP/MAS C-NMR study of cellulose structure in spruce wood and Kraft pulp. Cellulose 2000, 7, 35. (13) Weiping, B. Lucian A. L. Kinetic Profiling of Green LiquorModified Kraft Pulping. Ind. Eng. Chem. Res. 2005, 44, 2948. (14) Santiago, A. S.; Neto, C. P. Impact of Kraft Process Modifications on Eucalyptus globulus Pulping Performance and Polysaccharide Retention. Ind. Eng. Chem. Res. 2008, 47, 7433. (15) Vroom, K. E. The factor H: A means of expressing cooking times and temperatures as a single variable. Pulp Paper Mag. Can. 1957, 58, 228. (16) Sánchez, R.; Franco, J. M.; Delgado, M. A.; Valencia, C.; Gallegos, C. Thermal and mechanical characterization of cellulosic derivatives-based oleogels potentially applicable as bio-lubricating greases: Influence of ethyl cellulose molecular weight. Carbohydr. Polym. 2011, 83, 151. (17) Wise, L. E.; Murphy, M.; D̀ Adieco, A. Chlorite holocellulose, its fractionation and beating on summative wood analysis and on studies on the hemicellulose. Tech. Assoc. Pap. 1946, 29, 210. (18) Gericke, M.; Schlufter, K.; Liebert, T.; Heinze, T.; Budtova, T. Rheological properties of cellulose/ionic liquid solutions: from dilute to concentrated states. Biomacromolecules 2009, 10, 1188. (19) Kubes, G. J.; Fleming, B. I.; Macleod, J. M.; Bolker, H. I. Viscosities of unbleached pulps. II: The G-factor. J. Wood Chem. Techn. 1983, 3 (3), 313. (20) Lai, Y. Z. Chemical degradation. In Wood and cellulosic chemistry; Hon, D. N.-S., Shiraishi, N., Eds.; Marcel Dekker: New York, 2001; pp 443−512. (21) Gierer, J. Chemical aspect of kraft pulping. Wood Sci. Techn. 1981, 14, 241. (22) Ziaie-Shirkolaee, Y.; Mohammadi-Rovshandeh, J.; RezayatiCharani, P.; Khajeheian, M. B. Influence of dimethyl formamide pulping of wheat straw on cellulose degradation and comparasion with Kraft process. Bioresour. Technol. 2008, 99, 3568. (23) Douglas, A. F.; Lewis, G. A.; Spaull, A. J. B. The investigation of the dynamic viscoelastic functions of printing inks. Rheol. Acta 1971, 10, 382. (24) Mewis, J.; Spaull, A. J. B. Rheology of concentrated dispersions. Adv. Colloid Interface Sci. 1976, 6, 173. (25) Martín-Alfonso, J. E.; Valencia, C.; Sánchez, M. C.; Franco, J. M.; Gallegos, C. Evaluation of different polyolefins as rheology modifier additives in lubricating grease formulations. Mater. Chem. Phys. 2011, 128, 530. (26) Sánchez, M. C.; Franco, J. M.; Valencia, C.; Gallegos, C.; Urquiola, F.; Urchegui, R. Atomic force microscopy and thermorheological characterisation of lubricating greases. Tribol. Lett. 2011, 41, 463. (27) Almdal, K.; Dyre, J.; Hvidt, S.; Kramer, O. Towards a phenomenological definition of the term “gel”. Polym. Gels Networks. 1993, 1, 5. (28) Delgado, M. A.; Valencia, C.; Sánchez, M. C.; Franco, J. M.; Gallegos, C. Influence of soap concentration and oil viscosity on the rheology and microstructure of lubricating greases. Ind. Eng. Chem. Res. 2006, 45, 1902. (29) Franco, J. M.; Delgado, M. A.; Valencia, C.; Sánchez, M. C.; Gallegos, C. Mixing rheometry for studying the manufacture of lubricating greases. Chem. Eng. Sci. 2005, 60, 2409. (30) Delgado, M. A.; Sánchez, M. C.; Valencia, C.; Franco, J. M.; Gallegos, C. Thermorheological behaviour of a lithium lubricating grease. Tribol. Lett. 2006, 23, 47. (31) Sánchez, R.; Franco, J. M.; Delgado, M. A.; Valencia, C.; Gallegos, C. Development of new green lubricating greases formulations based on cellulosic derivates and castor oil. Green Chem. 2009, 11, 686.

In general, the values of linear viscoelastic functions increase with the H factor. Cellulose pulps submitted to lower or intermediate H factors provide gel-like dispersions with SAOS function values and thermal dependence comparable to those found in traditional lubricating greases. The rheological response of these cellulosic pulp-based gel-like dispersions studied is a consequence of the combined influence of DP and lignin and α-cellulose contents, which are affected by the H factor applied. Empirical linear correlations between both the plateau modulus and mechanical stability of gel-like dispersions and a power series of DP and α-cellulose and lignin weight fractions were established. Attending to both the rheological response and mechanical stability of these gel-like dispersions, the cellulose pulp submitted to intermediate cooking conditions (H = 300) can be proposed as the most suitable alternative thickener agent to formulate biodegradable lubricating greases.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34959219995. Fax: +34959219983. E-mail: franco@ uhu.es. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is part of a research project (CTQ2010-15338) sponsored by a MEC-FEDER program. One of the authors (N.N.) has received a Ph.D. Research Grant from the “Ministerio de Ciencia e Innovación”. The authors gratefully acknowledge its financial support. Thanks are also given to ENCE S.A. (Huelva factory) for the supply of industrial cellulose pulp samples.

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REFERENCES

(1) Rimdusit, S.; Jingjid, S.; Damrongsakkul, S.; Tiptipakorn, S.; Tackeichi, T. Biodegradability and property characterizations of Methyl Cellulose: Effect of nanocompositing and chemical crosslinking. Carbohydr. Polym. 2008, 72, 444. (2) Wilson, B. Lubricants and functional fluids from renewable sources. Ind. Lubric. Tribol. 1998, 50, 6. (3) Sacchettini, M.; Magnin, A.; Piau, J. M.; Pierrard, J. M. Caractérisation d’une graisse lubrifiante en écoulements viscosimétriques transitoires. J. Theor. Appl. Mech. 1985, 4, 165. (4) NLGI. Lubricating Greases Guide; National Lubricating Grease Institute: KS, 1994. (5) Balan, C. The rheology of lubricating greases; ELGI: Amsterdam, The Netherlands, 2000. (6) Park, J. S.; Ruckenstein, E. Viscoelastic properties of plasticized methylcellulose and chemically crosslinked methylcellulose. Carbohydr. Polym. 2001, 46, 373. (7) Núñez, N.; Martín-Alfonso, J. E.; Eugenio, M. E.; Valencia, C.; Díaz, M. J.; Franco, J. M. Preparation and characterization of gel-like dispersions based on cellulosic pulps and castor oil for lubricant applications. Ind. Eng. Chem. Res. 2011, 50, 5618. (8) Gustafson, R. R.; Sleicher, C. A.; McKean, W. T.; Finlayson, B. A. Theorical model of the Kraft pulping process. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 87. (9) Nguyen, K. L.; Dang, V. Q. The fractal nature of Kraft pulping kinetics applied to thin eucalyptus nitens chips. Carbohydr. Polym. 2006, 64, 104. (10) Hattula, T. Effect of Kraft cooking on the ultrastructure of wood cellulose. Paperi ja Puu 1986, 12, 926. (11) Newman, R. H.; Hemmingson, J. A.; Suckling, I. D. Carbon-13 nuclear magnetic resonance studies of Kraft pulping. Holzforschung 1993, 47, 234. 9782


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