Optimization of the Methylation Conditions of Kraft Cellulose Pulp for

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Ind. Eng. Chem. Res. 2009, 48, 6765–6771

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Optimization of the Methylation Conditions of Kraft Cellulose Pulp for Its Use As a Thickener Agent in Biodegradable Lubricating Greases J. E. Martı´n Alfonso, R. Yan˜ez, C. Valencia, J. M. Franco, and M. J. Dı´az* Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Experimentales, Campus de “El Carmen”, UniVersidad de HuelVa, 21071, HuelVa, Spain

This work evaluates the influence of the operating conditions used in the derivatization of commercial Kraft cellulose pulp (mercerization time (1-3 h) methylation process temperature (30-60 °C), methylation process time (1-4 h), (initial pulp)/(final wet alkali pulp) ratio (S/S) (1/7-1/12), and pulp/acetone ratio (S/L) concentration (1/3-1/12)) on the obtained methylcellulose pulp characteristics such as degree of polymerization of cellulose, substitution degree, water solubility, and viscosity of the resulting pulps in order to determine the best process conditions of this pulp for its use as a thickener agent in biodegradable lubricating grease formulations. Efficient use of the raw material (i.e., obtaining a degree of polymerization with suitable substitution degree) entailed using a 2.5 h methylation time and a 45 °C methylation temperature. The substitution degree (DS) and consistency index thus obtained are only 14.5% and 36.3% respectively lower than the optimum value, and an increment of 28.6% could be obtained for the degree of polymerization. Some methylcellulose-based oleogels display rheological properties very similar to those found in traditional lubricating greases. Methylcellulose with DS values of around 0.4 produces oleogels with better mechanical properties than highly derivatized cellulose and much better than the original Kraft cellulose pulp. 1. Introduction Nowadays, one of the main problems of very different industrial sectors concerns the impact that process technologies and products cause in the environment. In this sense, there is a general tendency to promote both the replacement of nonrenewable raw materials by renewable resources and the minimization of the environmental impact caused by industrial waste materials. Concerning the impact that lubricants exert on the environment, every year millions of tonnes of engine, industrial, and hydraulic oils are leaked into the ground or motorways or are poured into the environment. In order to minimize the contamination damage that petroleum-derived products cause, there has been an increasing demand for environmentally acceptable products suitable to be used as lubricants.1,2 Among them, lubricating greases constitute a group of lubricants with particular properties mainly related to their gel-like characteristics, which are imparted by the thickener agent (traditionally metallic soaps, phyllosilicates, or polyurea compounds). Little information regarding the development of biodegradable lubricating greases has been, up to now, available in the literature, and this is mainly focused on the substitution of mineral oil for a vegetable oil in the formulation.3 The substitution of traditional thickener agents by other more environmentally acceptable materials seems to present much more difficulty, mainly due to the functional properties and effectiveness achieved with those compounds, especially metallic soaps. Alternative thickeners coming from natural resources must impart to the final product, apart from nontoxicity and biodegradability, gel-like characteristics, thermal resistance, and efficiency to minimize wear in the machinery similar to that achieved with the traditional nonrenewable thickener agents.4 Obviously, the discovery of a biodegradable thickener agent with such suitable properties * To whom correspondence should be addressed. Phone: +34959219990. Fax: +34959219983. E-mail: [email protected].

may create a new market niche with interesting perspectives for the future.1 In this sense, products derived from cellulose, which is one of the most important substances obtained from renewable resources, may represent interesting alternatives to replace nonrenewable polymeric compounds and thickeners in general. Methylcellulose is a relatively common cellulose derivative with a very wide spectrum of applications in several industrial sectors related to building, surfactants, wallpaper paste, food, cosmetics, pharmaceuticals, detergents, polymers, etc.5-7 Two main types of commercial methylcelluloses are available: (a) water-soluble methylcelluloses with substitution degree (DS) values ranging from 1.4 to 2.0 (usually 1.8) and (b) alkali-soluble methylcelluloses with substitution degrees ranging from 0.25 to 1.0.5 This interesting property, i.e. the dissolution in water as a function of the substitution degree,8 could be the deciding factor for its application. The derivatization of cellulose fibers into methylcellulose in several substitution degrees7,9 in order to reduce the polarity of the original cellulose may represent, among another uses, an interesting way to achieve a polymeric biodegradable material, which could result in applications such as thickener agents in lubricating greases. The suitability of methylcellulose as a thickener in lubricant formulations is related, a priori, to the capacity to form a gel-like colloidal suspension in an oil medium. In this sense, molecular weight and DS seem to be important parameters. Taking into account these considerations, this work is focused on the development of new thickener agents based on methylcellulose derived from cellulose pulp in order to obtain 100% biodegradable lubricating grease formulations. Both the previous treatment (mercerization time) and derivatization process for kraft cellulose pulp were optimized. In particular, the effect of mercerization time, methylation process temperature, methylation process time, initial pulp/ final wet alkali pulp (pulp/alkali) ratio, and pulp/acetone ratio concentration on the obtained methylcellulose pulp characteristics have been investigated.

10.1021/ie9002766 CCC: $40.75  2009 American Chemical Society Published on Web 06/17/2009

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Table 1. Properties of Commercial Grade Kraft Pulp Used in This Study componenta R cellulose (%) hemicelluloses (as xylan) (%) lignin (%) intrinsic viscosity (SCAN value) (cm3 g-1) humidity (%) a

72.49 20.43 0.28 688 5

Oven-dried weight.

2. Experimental Section 2.1. Materials. Commercial grade Kraft pulp of eucalyptus sheets were kindly supplied by ENCE, S.A. (Huelva factory, Spain). Table 1 provides the composition and some physical properties of this pulp. The lignin and R-cellulose contents were analyzed according to Tappi procedures T-222 and T-203-OS61, respectively. Holocellulose contents were determined using the method described by Wise.10 The hemicellulose content (expressed as xylan) was determined by subtracting the amount of R-cellulose from that of the holocellulose sample. Pulp sheets were cut into pieces using scissors. These pieces were disintegrated using a miller Heidolph RZR 2051 control model. All chemicals were supplied for Sigma-Aldrich Co. as reagent grade. Castor oil (211 cSt at 40 °C, Guinama, Spain) was used as biodegradable lubricating oil to prepare Oleogel formulations with methylcellulose samples. Cellulose and methylcellulose were dispersed in castor oil (7% w/w) at room temperature using an anchor impeller geometry (50 rpm for 45 min). 2.2. Methylation of the Kraft Bleached Cellulose (KBC). The methylation of the Kraft bleached cellulose was made according to the method described by Mansour et al.11 15 g pulp and 375 g of 50% NaOH solution were used for the mercerization. These blends were homogenized and lasted for desirable time at room temperature. After the mercerization, the pulp was filtered and pressed to a pulp/alkali ratio from 1/7 to 1/12 (w/w). Then, the mercerized pulp together with both 45 mL of dimethyl sulfate, DMS, (3 mL/1 g KBC) and a variable amount of acetone was added to a flask of 500 mL and submitted to agitation. The mixture was left in a water bath at a temperature between 30-60 °C and for 1-4 h following the proposed experimental design, being occasionally stirred. At the end of the reaction, the material was neutralized with a 10% acetic acid solution, filtered (0.5 mm), and washed with acetone. The methylcellulose was dried at room conditions to constant weight. 2.3. Experimental Design for the Methylation of Kraft Bleached Cellulose. In order to be able to relate the dependent and independent variables with the minimum possible number of experiments, an orthogonal main effect design consisting of a central point (central experiment, in the center of a cube, duplicated) and 16 additional points (additional experiments lying at the cube vertices) was used. This experimental design that enabled the construction of first-order polynomials in the independent variables and the identification of statistical significance in the variables was used.12,13 The polynomial model used was of the following type: n

Z ) a0 +

∑ i)1

n

biXni +

∑

diXniXnj (i < j)

(1)

i)1;j)1

where Z and Xni denote dependent and normalized independent variables, respectively, and a0, bi, ci, and dij are unknown constants obtained from experimental data. Independent variables were normalized (Xn) by using the following equation:

Xn ) (X - Xmed)/[(Xmax-Xmin)/2]

(2)

Where, X is the absolute value of the independent variable concerned, Xmed is the average value of the variable, and Xmax and Xmin are their maximum and minimum values, respectively. The independent variables used in the equations relating to both types of variables were those having a statistically significant coefficient (viz. those not exceeding a significance level of 0.05 in student’s t test and having a 95% confidence interval excluding zero). The range of values for each independent variable (mercerization time, methylation process temperature, methylation process time, pulp/alkali ratio, pulp/acetone ratio) used in the proposed experimental design is shown in Table 2. The properties of cellulose/methylcellulose pulp selected as dependent variables for each model are the degree of polymerization of cellulose (DP), substitution degree (DS), water solubility (WS), and the consistency index of cellulose/NaOH solutions (k). 2.4. Characterization of Methylcelluloses. The methylcellulose pulp moisture content was determined by drying to a constant weight according to Tappi T-257. The degree of polymerization of cellulose have been deduced from viscosity (V) of pulp (T-230) using eq 3. SCAN-C15 official method was used to correlate the viscosity obtained (T-230, mPa s) with the intrinsic viscosity (cm3/g) used in this equation. DP0.905 ) 0.75 v

(3)

Equation 3, in this study, when it is applied to methylcellulose, is used as an approximation. Water solubility was determined by stirring 0.3 g of the methylcellulose in 25 mL of distilled water and filtering the solution through membranes of 5 m pore diameter. The final weight was determined as total solids dried at 100 °C until a constant weight was obtained. Determination of the degree of substitution was carried according to the method described by Daiyong.14 Methylcellulose was dissolved in distilled water in excess of NaOH, and the solution is submitted to a conductometric titration with a HCl solution. 2.5. Rheological Tests. Viscous properties of methylcellulose solutions in 4% NaOH were determined in a controlledstress rheometer RS-150 (ThermoHaake, Germany) at 25 °C, using a coaxial cylinder sensor system (radii ratio, Re/Ri ) 1.085) in a shear stress range of 0.01-20 Pa. In all cases, a non-Newtonian behavior was detected which can be satisfactorily described (R2 > 0.950) by the power-law model in the shear rate range studied: η ) kγ˙ n-1

(4)

where η is the apparent viscosity, γ˙ is the shear rate, and “k” and “n” are the consistency and viscous flow indexes, respectively. Rheological characterization of biodegradable lubricating greases was carried out with a Rheoscope controlled-stress rheometer (ThermoHaake, Germany). Small-amplitude oscillatory shear (SAOS) tests were performed inside the linear viscoelastic region, using a plate-plate geometry (20 mm, 1 mm gap), in a frequency range of 10-2-102 rad/s. At least two replicates of each test were performed on fresh samples. 2.6. Penetration Tests. Penetration indexes and NLGI consistency numbers of biodegradable lubricating greases tested were determined according to the ASTM D 1403 standard, by

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using a Seta Universal penetrometer, model 17000-2, with onequarter cone geometry (Stanhope-Seta, UK). The one-quarter scale penetration values were converted into the equivalent fullscale cone penetration values, following the ASTM D 217 standard. 3. Results and Discussion 3.1. Methylation Process Optimization. References about factors that can influence the methylation process (in heterogeneous or homogeneous media) have been reported by several authors.9,15-17 Among the variables studied by these authors, those that have shown more influence on the methylcellulose synthesis are the following: mercerization time, methylation temperature, methylation time, cellulose/dissolvent ratio, and cellulose/alkali cellulose ratio.11,15,18 Nevertheless, models which take into account the influence of more than two variables simultaneously have not been found. To obtain such models, the normalized values of independent variables and properties of the cellulose-methylcellulose pulp obtained using the proposed experimental design (Table 3) have been correlated. Each value of these properties is an average of three experimental results. The deviations for these parameters from their respective means were all less than 10%. The values of the flow index for cellulose/NaOH solutions (eq 4) are very similar in all cases (n ) 0.15 ( 0.07) and are not significantly influenced by the studied variables. Substituting the values of the independent variables for each dependent variable in Table 3 into the polynomial expression used yielded the following equations: DP ) 443.76 - 50.40XMRT - 97.13XT - 49.38XMTT + 41.61XS/S + 46.61XMRTXMTT - 34.51XMRTXS/S 2 (R ) 0.92; F ) 34.9; degrees of freedom (df) ) 6.11)

(5)

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DS ) 0.337 - 0.077XT + 0.094XS/L - 0.127XMTT 0.132XMRTXT - 0.055XMRTXS/L + 0.075XTXMTT (R2 ) 0.82; F ) 6.11; df ) 12.76)

(6)

WS ) 23.21 + 4.31XMRT+4.82XS/L - 5.20XMTT 3.31XMRTXT - 4.32XMRTXMTT + 2.325XTXS/L 2.96XMTTXS/L (R2 ) 0.85; F ) 7.10; df ) 15.06)

(7)

k ) 0.83 + 0.509XT + 0.362XMRT + 0.219XMRTXT + 0.391XMRTXS/S (R2 ) 0.79; F ) 5.4; df ) 4.13)

(8)

Equations 5-8 reproduce the degree of polymerization of cellulose, substitution degree, water solubility (%), and consistency index of the cellulose/NaOH solution (Pa sn) respectively of the obtained pulp. XMRT, XT, XMTT, XS/S, and XS/L denote the normalized values of the mercerization time, methylation process temperature, methylation process time, pulp/alkali ratio (S/S), and pulp/acetone ratio (S/L), respectively. The differences between the experimental values and those that were estimated using the previous equations never exceeded 10% of the former, with except to for the consistency index of the cellulose/NaOH solution that the differences reached to 23%. The pulp/acetone ratio in eqs 5 and 8 and the pulp/alkali ratio in eqs 5 and 7 did not have a statistically significant coefficient over the ranges considered. Moreover, identifying the independent variables most and least strongly influencing the dependent variables in eqs 5 to 8 is not so easy since the former contain interactions between two independent variable terms Figure 1 shows a plot of each dependent variable against each independent one constructed by changing all the independent variables between the normalized values from -1 to +1. At a given value of an independent variable, the magnitude of the difference between the maximum and minimum values of the dependent variable is related to the influence of the independent variables other than that plotted on the variation of the dependent variable concerned. Thus, if the independent variable plotted

Table 2. Range of Each Independent Variable Used in the Proposed Experimental Design mercerization time (h)

methylation process temperature (°C)

methylation process time (h)

pulp/alkali ratio (S/S) (kg/kg)

pulp/acetone ratio (S/L) (kg/L)

nomalized value

1.0 2.0 3.0

30 45 60

1.0 2.5 4.0

1/7 1/9 1/12

1/3 1/7 1/12

(-1) (0) (+1)

Table 3. Values of Independent Variables and Properties of the Pulp Obtained Using the Proposed Experimental Design normalized values of mercerization time, methylation process temperature, methylation process time, pulp/alkali ratio (S/S), and pulp/acetone ratio (S/L)

degree of polymerization of cellulose (DP)

substitution degree (DS)

water solubility (WS, %)

consistency index (k, Pa sn)

-1, -1, -1, -1, 1 -1, -1, -1, 1, -1 -1, -1, 1, -1, -1 -1, -1, 1, 1, 1 -1, 1, -1, -1, -1 -1, 1, -1, 1, 1 -1, 1, 1, -1, 1 - 1, 1, 1, 1, -1 1, -1, -1, -1, -1 1, -1, -1, 1, 1 1, -1, 1, -1, 1 1, -1, 1, 1, -1 1, 1, -1, -1, 1 1, 1, -1, 1, -1 1, 1, 1, -1, -1 1, 1, 1, 1, 1 0, 0, 0, 0, 0 0, 0, 0, 0, 0

791.8 398.7 629.5 586.8 410.5 373.9 553.5 258.0 511.4 542.9 472.5 442.9 336.3 326.2 289.1 274.9 394.3 396.2

0.34 0.01 0.43 0.20 0.16 0.10 0.70 0.47 0.82 0.23 0.88 0.41 0.20 0.10 0.19 0.18 0.34 0.32

12.8 11.8 22.4 14.3 14.2 16.8 31.3 30.8 37.6 19.6 46.1 19.0 19.5 18.0 46.6 17.0 20.0 21.0

0.49 0.53 0.14 0.42 1.12 2.26 0.09 0.42 0.06 0.85 0.03 0.13 0.55 1.35 1.35 3.65 0.04 1.51

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Figure 1. Variation of dependent variables as a function of normalized independent variables.

had no effect, then the difference between the maximum and minimum values of the dependent variable in question would coincide with the height of the rectangle having the range of values of the independent variable plotted, [(Xni)max - (Xni)min], and the maximum possible difference between the maximum and minimum values of the dependent variable considered, {Z[(Xni)max]max - Z[(Xni)min]min}, as its bases. Because the influence of the other variables on the dependent variable considered can vary with each value of the independent variable plotted, the average change in the dependent variable will be given by

∫

(Xni)max

(Xni)min

[Z(Xni)max - Z(Xni)min] dXni

[(Xni)max - (Xni)min]

(9)

The change in the dependent variable with that in the independent variable can be plotted to express the difference between [Z(Xni)max]max - Z[(Xni)min]min and the previous expression: DZi ) {[Z(Xni)max]max - [Z(Xni)min]min} -

∫

(Xni)max

(Xni)min

[Z(Xni)max - Z(Xni)min] dXni

[(Xni)max - (Xni)min]

(10)

Figure 1 also shows the DZi values (in brackets). These values allow one to weigh the relative influences, as percentages, of each independent variable on the variation of each dependent variable. As can be seen, the methylation temperature is the variable most strongly influencing degree of polymerization of cellulose whereas methylation reaction time is that having the strongest effect on substitution degree and both, mercerization time and S/L, are that having the main effect on water solubility. In order to determine the values of the independent variables giving the optimum DP, DS, WS, and k, the response surfaces for each dependent variable were plotted at three extreme levels of the independent variable most strongly influencing each and a fixed value of the least influential variable (Figures 2-5). Degree of polymerization of cellulose (eq 5) was found to depend mainly on methylation temperature, and less significant influences on mercerization and methylation time have

Figure 2. Degree of polymerization of cellulose variation as a function of mercerization and methylation time at three methylation temperature levels.

been found. The DP (Figure 2) value indicates the relative degradation of cellulose fiber during the derivatization process, and therefore, the results of DP can be applied in estimating the extent of cellulose degradation during derivatization process. DP values obviously decreased after high methylation temperature. For the same treatment temperature, DP had significantly lower values under high values of methylation time and mercerization time than that treated under lower values of those variables. The DP decreased mainly with increasing treatment time. Higher degradation was found for cellulose after a high mercerization time. Long mercerization time did not produce high DP decrease under short methylation time; however, a significant descent could be found under high methylation time. Thus, maximizing the

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Figure 3. Substitution degree variation as a function of mercerization time and methylation temperature at three methylation time levels.

DP entails using low or medium methylation temperature (30-45 °C), methylation time (1 h), and mercerization time (1 h). The strength values of these methylcellulose fibers are highly and inversely dependent on the type, substitution degree (DS), and degree of polymerization of cellulose.19 Nevertheless, a high DS is desirable to obtain a high methylation level, and then, a higher substitution degree could be the most adequate value. The DS (Figure 3) was much more sensitive to changes in methylation time and mercerization time than to those in methylation temperature; hence, the greatest changes in DS resulted from variation of the methylation time (84.4% with respect to the maximum value) and mercerization time (60.3%). In that form, an increment in mercerization time produces a descent in the substitution degree; on the contrary, an increment in methylation temperature produces an increase in the substitution degree. As can be seen in Figure 3, obtaining the optimum DS entails using a long methylation time (4 h), short mercerization time (1 h), and high methylation temperature (60 °C). If the primary goal is to maximize DSsby exploiting the whole potential of the raw material in its use as thickener agent in biodegradable lubricating greasesswhile saving in equipment immobilized capitalsby reducing the methylation time (2.5 h) and temperature (45 °C)sone can use a medium methylation temperature and short mercerization time. The DS thus obtained is only 14.5% lower than the optimum value. The water solubility (WS) of the methycellulose is imparted by the hydrophilic groups such as the residual hydroxy group and the oxygen at the ether and acetal linkages.19 Moreover, the WS depends on how uniform the methoxyl groups are distributed along the polymeric chain and the substitution degree.11 Cellulose is usually modified in heterogeneous conditions due to its insolubility in most usual solvents. Under those conditions, the distribution of the substituted groups is not uniform.7 The difference between the water properties of the methylcellulose that is produced according to each route is related to the formation of a gel fraction in aqueous medium. In this study, it should be noted in Figure 4 that the variation of mercerization time had the most influence on rising the WS at all levels of S/L ratio and methylation time conditions. It

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Figure 4. Water solubility variation as a function of methylation time and pulp/acetone ratio at three mercerization time levels.

Figure 5. Consistency index variation as a function of methylation temperature and pulp/alkali ratio at three methylation time levels.

was much more sensitive to changes in S/L than in methylation time at all levels of mercerization time. Additionally, the response surface of WS shows that it could be decreased under higher mercerization time and, only slightly, with methylation time and pulp acetone ratio. Under lower mercerization time (1 h), methylation time (1 h), and S/L ratio (1/12), high values for WS could be obtained. The viscous flow behavior of methylcellulose pulp was measure in 4% NaOH solution. The consistency index (k) (Figure 5) of this solution is a parameter related to the consistency of the sample, i.e. the apparent viscosity at 1 s-1, and may be an important factor that correlates with the service properties of lubricating greases. In most cases, greases that are excessively softened during service may be leaked from the application and cause failure in the equipment. On the contrary, more consistent greases allow the lubricant to be kept in place, prevent the loss of lubricant under operating conditions, avoid the penetration of contaminants, such as solid particles or water,

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as thermal dependence, is needed to test the real applicability of these biolubricating greases. 4. Conclusions

Figure 6. Frequency dependence of the storage and loss modulus at 25 °C, in the linear viscoelasticity region for oleogels. Table 4. Penetration Values and NLGI Consistency Numbers for Different Oleogels Prepared with Methylcellulose or Cellulose Pulps and Castor Oil thickener (7% w/w)

penetration index (dmm)

NLGI grade

Kraft cellulose pulp methylcellulose (DS: 0.43) methylcellulose (DS: 0.88)

258 253 246

2-3 2-3 3

and act as a seal without a significant reduction of its lubricants properties. In that form, high k values could be desirable to obtain optimum biolubricating greases from methylcellulose derivates. From Figure 5, it follows that the methylation time has a more marked effect on the k values than the other independent variables. However, no differences in k values under low methylation temperature (30 °C) and a divergence in this value at high methylation temperature have been found. Similar evolution with respect to the S/S ratio among the other independent variables values is shown. Thus, obtaining the optimum levels of k entailed using a high methylation temperature (60 °C) and S/S ratio (1/7). 3.2. Rheological Properties of Methylcellulose-Based Oleogels. Some oleogel formulations potentially applicable as biodegradable lubricating greases were prepared using selected samples of original and derivatized Kraft cellulose pulps as thickener agents. Figure 6 and Table 4 show the linear viscoelastic response and consistency data of these oleogels, respectively. Original Kraft cellulose pulp-based oleogel presents two important disadvantages in relation to traditional lubricating greases. First, this oleogel may be considered a highly strong gel, with values of the SAOS functions (Figure 6) more than one decade higher than traditional lubricating greases.20-22 To continue and more important, a significant oil phase separation is apparent (oil bleeding), even under static conditions, probably due to the rigidity of the gel network. However, stable oleogels with consistency numbers (NLGI grades 2-3) of most commonly employed traditional greases23 can be achieved by using methylcellulose with different DS values (see Table 4). Moreover, methylcellulose-based greases display mechanical spectra (Figure 6) qualitatively and quantitatively similar to those found in other model lubricating greases manufactured with mineral oils, lithium soap, and polymeric additives,24-27 especially in the case of methylcellulose with a DS of 0.434. This is a promising result although, evidently, much more work concerning mechanical and chemical stability, as well

Among all the studied reaction parameters, methylation temperature and time show the most significant influence on the physicochemical properties of methylcellulose obtained from cellulose Kraft. According to experimental results, we can obtain the optimum substitution degree and consistency index by using long methylation time (4 h), short mercerization time (1 h), and high methylation temperature (60 °C). However, low degree of polymerization of cellulose values could be obtained under those conditions. Efficient use of the raw material (i.e., obtaining a degree of polymerization with suitable substitution degree) entailed using a 2.5 h methylation time and a 45 °C methylation temperature. The substitution degree and consistency index thus obtained are only 14.5% and 36.3% respectively lower than the optimum value, and an increment of 28.6% on the degree of polymerization could be obtained. The linear viscoelastic response and NLGI grade of oleogels prepared with selected samples of methylcellulose are quite similar to those found in traditional lubricating greases. Methylcellulose with DS values of around 0.4 produces oleogels with better mechanical properties than highly derivatized cellulose and much better than the original Kraft cellulose pulp. Acknowledgment This work is part of a research project (CTQ2007-60463/ PPQ) sponsored by a MEC-FEDER Programme. The authors gratefully acknowledge their financial support. Thanks are also given to ENCE (Huelva, Spain) for the supply of raw cellulose samples. Nomenclature DP ) degree of polymerization of cellulose DS ) substitution degree WS ) water solubility (%) k ) consistency index (Pa sn) n ) flow index G′ ) storage modulus (Pa) G′′ ) loss modulus (Pa) XMRT ) normalized values of the mercerization time XT ) normalized values of the methylation process temperature XMTT ) normalized values of the methylation process time XS/S ) normalized values of the pulp/alkali ratio (S/S) XS/L ) normalized values of the pulp/acetone ratio (S/L)

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ReceiVed for reView February 18, 2009 ReVised manuscript receiVed May 28, 2009 Accepted June 2, 2009 IE9002766