Journal of Industrial and Engineering Chemistry 17 (2011) 705–711

Rheological and mechanical properties of oleogels based on castor oil andcellulosic derivatives potentially applicable as bio-lubricating greases:Influence of cellulosic derivatives concentration ratio

R. Sanchez, J.M. Franco *, M.A. Delgado, C. Valencia, C. Gallegos

Departamento de Ingenierıa Quımica, Facultad de Ciencias Experimentales, Campus de ‘‘El Carmen’’, Universidad de Huelva, 21071 Huelva, Spain

A R T I C L E I N F O

Article history:

Received 9 November 2010

Accepted 5 January 2011

Available online 13 May 2011

Keywords:

Cellulosic derivatives

Ethyl cellulose

Lubricating grease

Oleogel

Rheology

A B S T R A C T

Nowadays the lubricating market is demanding new biodegradable or more environmentally acceptable

products based on renewable resources as a consequence of progressively more strict environmental

regulations. In this framework, this study deals with the design of gel-like dispersions potentially

applicable as environmentally friendly lubricating greases. These dispersions were formulated using

castor oil and ethyl cellulose/a-cellulose or ethyl cellulose/methyl cellulose blends. In particular, the

influence of cellulosic derivatives concentration ratio on the linear viscoelasticity and mechanical

stability of the resulting oleogel formulations was studied. The modification of ethyl cellulose/a-

cellulose or ethyl cellulose/methyl cellulose weight ratios allows obtaining some formulations with

suitable rheological characteristics and mechanical stability for potential lubricating applications. An

important decrease in the values of the linear viscoelasticity functions down to a minimum value was

found by increasing ethyl cellulose/a-cellulose or ethyl cellulose/methyl cellulose weight ratios (W) up

to a critical value, which depends on both nature of the cellulosic derivatives employed and temperature.

Above this critical value, the linear viscoelastic functions increase with W, at temperatures in the range

0–75 8C, and continuously decrease at higher temperatures, i.e. 125 8C. Thermal susceptibility is

significantly dampened by reducing ethyl cellulose concentration. Gel-like dispersions formulated with

ethyl cellulose/methyl cellulose blends showed appropriate mechanical stabilities to be used as bio-

lubricating greases.

� 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Lubricating greases are highly structured suspensions, tradi-tionally consisting of a thickener dispersed in mineral or syntheticoil [1,2]. Fatty acid soaps of lithium, calcium, sodium, aluminium orbarium are most commonly employed as thickener agents.Although the lubricating component is the base oil, the thickeneris a key component, added to increase the consistency of thelubricant, in order to solve some difficulties that lubricating oilscannot cover properly in specific applications [3]. With thispurpose, during the manufacturing process, the thickener isinduced to form a three-dimensional network in the oil medium,which imparts to the lubricating grease the desired gel-likecharacteristics and, specifically, appropriate rheological properties[4–6]. Thus, for instance, optimal rheological properties help notonly to prevent loss of lubricant under operating conditions whenthey are subjected to inertial forces and shear, but also to avoid the

* Corresponding author. Tel.: +34 959219995; fax: +34 959219983.

E-mail address: franco@uhu.es (J.M. Franco).

1226-086X/$ – see front matter � 2011 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2011.05.019

penetration of contaminants (solid particles, water, etc.). In thissense, the grease plays the role of a seal without a significantreduction of its lubricating properties, since only a small part ofgrease acts as a lubricant in the contact between differentmechanical parts in relative motion [7].

Operating conditions in current technologies impose highdemands on lubricating greases [8,9]. For instance, they need toresist high and low temperatures, high pressures or adverseatmospheres where the properties of lubricating greases can bemodified drastically. As a result, they may lose the ability to favouran appropriate lubrication. Nowadays, the lubricating market isinterested not only in materials that can be used in theseconditions but it is also demanding new completely biodegradableproducts, or more environmentally acceptable, than the traditionallubricants, even although this new type of products will implyhigher prices. The main reason for the tendency of replacing non-renewable raw materials by renewable resources in the field oflubricants is due to the high impact that they exert on theenvironment, since, every year, millions of tonnes of engine,industrial and hydraulic oils are leaked into the ground or pouredin the environment. This is a rather problematic issue, as small

ing Chemistry. Published by Elsevier B.V. All rights reserved.

R. Sanchez et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 705–711706

amounts of these products can inhibit the growth of trees and canbe toxic to aquatic life [10].

On the other hand, motivations to design new biodegradablematerials are also induced by government incentives, publicopinion, corporate programs of social and environmental respon-sibility, and corporate response to marketing needs. To reach thisobjective, lubricants producers are looking for suitable materials toprovide adequate biodegradable products. This idea is not reallynew, since formulations with, at least, 70–80% degradabilityrequirements of the basic ingredient (oil) appeared in thelubricants market several decades ago (for instance, the BlueAngel eco-label in 1978) [11]. However, the objective of developingenvironmentally compatible lubricants contains a number ofadditional requirements, such as classification in water hazardcategory ‘‘0’’, and no content of heavy metals, halogenatedhydrocarbons or nitrite, taking also into account that the mostimportant requirement is that grease performance must not beaffected negatively [12]. Moreover, 100% biodegradable materialsare necessary to cover some specific lubrication problems, i.e.lubrication in food production, aiming to prevent any type ofcontamination in the manufacture of the final product. In thissense, the United States government has classified lubricants usedin food processing plants into two categories: (i) H1, lubricantsused in applications where there is a possibility of incidental foodcontact; and (ii) H2, lubricants used in applications where there isnot possibility of food contact [13]. Some biodegradable greasesconsisting of vegetable oils, synthetic esters or glycols as basefluids; metal soaps or polyureas as thickeners, and additives toimprove the behaviour of the final material in specific conditions,have been designed to cover this goal [14]. However, this generalformulation still includes highly non-biodegradable components,particularly the thickener agents, and, consequently, more effortsto find new suitable ingredients to manufacture a completelybiodegradable material are needed.

Biodegradable polymers could be used to reduce the environ-mental problems in many different areas (for example plasticwaste in daily use [15]) since they have low manufacturing costsand easy processibility in large-scale production. Among thesebiopolymers, cellulose and its derivatives are the most attractiveoptions mainly due to the different applications where thesematerials could play an important role [16–18]. Biopolymericadditives used in industrial formulations should also presentstability against mechanical degradation [19]. In previous studies[20,21], gel-like dispersions based on castor oil and cellulosicderivatives were proposed as potential biodegradable lubricatinggreases. Particularly, the use of ethyl cellulose combined withother cellulosic derivatives, i.e. a-cellulose or methyl cellulose,provides an appropriate thermal and rheological behaviour of thelubricating grease. In this work, the influence of cellulosicderivatives concentration ratio on the linear viscoelasticity andmechanical stability of the resulting formulations was studied.

Table 1Chemical compositions of the oleogel formulations studied.

Castor oil (%) Ethyl cellulose (EC) (%) a-Cellulose

78.0 1.2 20.8

78.0 1.6 20.4

78.0 2.0 20.0

78.0 2.4 19.6

78.0 2.7 19.3

73.0 1.0 –

73.0 1.5 –

73.0 2.0 –

73.0 2.5 –

73.0 2.9 –

2. Experimental

2.1. Materials

Castor oil (211 cSt at 40 8C, Guinama, Spain) was selected asbiodegradable lubricating oil. Ethyl cellulose (Mn: 60,000 g/mol;48% ethoxy content), methyl cellulose (Mn: 40,000 g/mol; 32%methoxy content), and a-cellulose, all of them from Sigma–Aldrich, were used as gelling agents to prepare different gel-likedispersions. Standard lithium 12-hydroxystearate lubricatinggreases (14–20% lithium soap) were used as reference systems.12-Hydroxystearic acid, lithium hydroxide, and paraffinic (334 cStat 40 8C) and naphthenic (115 cSt at 40 8C) oils were kindlysupplied by Verkol Lubricantes S.A. (Spain).

2.2. Manufacture of oleogel formulations

Gel-like dispersions (400 g batches) were processed in anopen vessel, using a helical ribbon impeller geometry(D = 90 mm; H = 90 mm) to disperse the gelling agents. In thefirst step, the vessel was filled with the corresponding amount ofcastor oil and ethyl cellulose. After that, a constant rotationalspeed (60 rpm) was applied to the mixture, at 150 8C, until ethylcellulose was completely dissolved and, then, the othercellulosic derivative was added, keeping the same rotationalspeed and temperature during 30 min. Finally, the mixture wascooled down to room temperature by natural convection. Table1 shows the composition of the different oleogel formulationsstudied. W values in Table 1 correspond to the ratios betweenthe concentrations of ethyl cellulose and the other cellulosicderivatives used as thickeners, i.e. a-cellulose or methylcellulose, respectively.

2.3. Rheological characterization

Rheological characterization of oleogels was carried out withtwo controlled-stress rheometers (RS-150 and Rheoscope, Ther-moHaake, Germany). Small-amplitude oscillatory shear (SAOS)tests were performed inside the linear viscoelastic region, using aplate–plate geometry (35 mm, 1 mm gap), in a frequency range of10�2–102 rad/s, and temperatures comprised between 0 and125 8C. A thermostatic recirculation water bath coupled withthe Rheoscope rheometer was used at lower temperatures whilean electric oven coupled with the RS-150 rheometer was employedat temperatures higher than 75 8C. At least two replicates of eachtest were performed on fresh samples.

2.4. Penetration and mechanical stability tests

Both unworked and worked penetration indexes were deter-mined according to the ASTM D 1403 standard, by using a Seta

(aC) (%) Methyl cellulose

(MC) (%)

(EC/aC) or (EC/MC)

weight ratio (W)

– 0.06

– 0.08

– 0.10

– 0.12

– 0.14

26.0 0.04

25.5 0.06

25.0 0.08

24.5 0.10

24.1 0.12

104

105

106

107

G

' G''

(Pa)

0 ºC 0 ºC

10-2

10-1

100

W = 0.06 W = 0.08 W = 0.10 W = 0.12 W = 0.14

tan δ

10-2 10-1 100 101 102103

104

105

G' G

'' (P

a)

ω (r ad/s)

125 ºC

103

104

105

106

G' G

'' (P

a)

25 ºC

10-2

10-1

tan δ

25 ºC

10-2 10-1 100 101 102 10310-2

10-1 tan δ

ω (r ad/s)

125 ºC103

104

105

G' G

'' (P

a)

75 ºC

10-2

10-1

tan δ

75 ºC

Fig. 1. Evolution of the storage modulus (G0 , filled symbols), loss modulus (G00 , open symbols) and loss tangent (tan d, half-filled symbols) with frequency, for oleogels

formulated with castor oil and a-cellulose/ethyl cellulose blends, at different temperatures.

R. Sanchez et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 705–711 707

Universal penetrometer, model 17000-2 (Stanhope-Seta, UK), withone-quarter cone geometry. The one-quarter scale penetrationvalues were converted into the equivalent full-scale conepenetration values, following the ASTM D 217 standard. The samesamples were worked during 2 hurs in a Roll Stability Tester,

model 19400-3 (Stanhope-Seta, UK), according to the ASTM D 1831standard. Penetration measurements were carried out, once again,immediately after this rolling test. Sample mechanical stabilitywas then calculated as the difference between worked andunworked penetration values.

R. Sanchez et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 705–711708

3. Results and discussion

Fig. 1 shows the evolution of the linear viscoelasticity functionswith frequency, at different temperatures, for gel-like dispersionsprocessed using castor oil and different combinations of a-cellulose and ethyl cellulose. All the formulations contain the same

104

105

106

G' G

'' (P

a)

0 ºC

10-2 10-1 100 101 102102

103

104

105

G' G

'' (P

a)

ω (rad/s)

125 ºC

103

104

105

G' G

'' (P

a)

25 ºC

10-2

102

103

104

105

G' G

'' (P

a)

75 ºC

Fig. 2. Evolution of the storage modulus (G0 , filled symbols), loss modulus (G00 , open s

formulated with castor oil and methylcellulose/ethyl cellulose blends, at different tem

total amount of cellulosic derivatives but different ethyl cellulose/a-cellulose weight ratios (W). The evolution of SAOS functions isvery similar to that found for standard lubricating greases [5,22].Thus, G0 values are always higher than G00 values in the wholefrequency range studied, and the ‘‘plateau’’ region of themechanical spectrum is always noticed, i.e. G0 slightly increases

10-2

10-1

tan δ

75 ºC

10-2

10-1

100

W = 0.04 W = 0.06 W = 0.08 W = 0.10 W = 0.12

tan δ

0 ºC

10-1 100 101 102 10310-2

10-1

tan δ

ω (rad/ s)

125 ºC

10-2

10-1

tan δ

25 ºC

ymbols) and loss tangent (tan d, half-filled symbols) with frequency, for oleogels

peratures.

Fig. 3. Evolution of the ‘‘plateau’’ modulus with cellulosic derivatives concentration

ratio, at different temperatures, for oleogels formulated with: (a) a-cellulose/ethyl

cellulose blends and (b) methylcellulose/ethyl cellulose blends. Fig. 4. Evolution of the viscosity with shear rate for ethyl cellulose/castor oil binary

systems corresponding to methylcellulose dispersions, at 25 8C (full symbols) and

125 8C (open symbols).

R. Sanchez et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 705–711 709

with frequency and G00 displays a minimum at intermediatefrequencies. This mechanical spectrum corresponds with thetraditional description given for solid-like gels [23]. As extensivelyinvestigated [24–27], G0 values in conventional lithium lubricatinggreases typically range from 104 to 105 Pa, at 25–75 8C, around oneorder of magnitude higher than G00 values, depending oncomposition and processing conditions. As can be observed inFig. 1, dispersions with the lowest ethyl cellulose/a-celluloseweight ratios (W = 0.06–0.08) show significantly higher values ofboth G0 and G00 and lower values of the loss tangent (tan d = G00/G0),especially at high frequencies, indicating an enhanced relativeelastic response. A dramatic decrease in both SAOS functions, butmore important in G0, is noticed by slightly increasing ethylcellulose/a-cellulose weight ratio (W = 0.10), in the wholetemperature range studied. On the other hand, the minimum inG00 and, consequently, in the loss tangent is generally shifted tolower frequencies. However, a further increase in W produces theopposite effect, at temperatures ranging from 0 8C to 75 8C.Consequently, SAOS functions increase, although this behaviour isnot associated with changes in the relative elasticity (see tan d inFig. 1). It is worth noting that this evolution of the linearviscoelasticity functions with W (0.10–0.14) is not found at 125 8C.At this temperature, a continuous decrease in the values of bothmoduli with ethyl cellulose/a-cellulose weight ratio is alwaysobserved. Moreover, the relative elasticity of the samples generallydecreases by increasing W.

Fig. 2 shows the evolution of the SAOS functions with frequencyfor gel-like dispersions containing methyl cellulose instead of a-cellulose. The influence of ethyl cellulose/methyl cellulose weightratio on the linear viscoelastic response of this type of dispersionsis very similar to that previously shown for oleogels containingethyl cellulose/a-cellulose blends. Thus, minimum values in bothG0 and G00 were found for an intermediate ethyl cellulose/methylcellulose weight ratio (W between 0.06 and 0.10, depending ontemperature), excepting at 125 8C, where SAOS functions continu-ously decrease with W. Moreover, oleogel relative elasticityincreases as ethyl cellulose/methyl cellulose weight ratiodecreases.

Fig. 3 summarizes and compares the effect of thickeners weightratio on the linear viscoelastic response of both a-cellulose- andmethyl cellulose-based oleogels. In this figure, the ‘‘plateau’’modulus evolution with W is presented. The plateau modulus, G0

N,is a characteristic parameter of the ‘‘plateau’’ region in themechanical spectrum, as described elsewhere [28]. Differentmethods for the determination of the plateau modulus have been

used [29]. A straightforward method to estimate G0N from the loss

tangent was selected in this work [1]:

G0N ¼ ½G0�tan d ! minimum (1)

As can be observed, G0N values reach minimum values at

intermediate values of W, excepting at 125 8C where a continuousdecrease of this viscoelastic parameter with W is clearly observed.The peculiar influence of W on SAOS functions, at 125 8C, is relatedto the sol–gel transition of ethyl cellulose in castor oil, which takesplace around 70 8C [21]. As previously described [20], ethylcellulose stabilizes the gel-like dispersion by significantly increas-ing castor oil viscosity below this critical temperature, thusreducing oil bleeding. Fig. 4 illustrates the evolution of the viscositywith ethyl cellulose concentration for ethyl cellulose/castor oilbinary systems corresponding to methyl cellulose dispersions, at25 and 125 8C. An important increase in viscosity, associated to anon-Newtonian behaviour, can be clearly observed at 25 8C. Thisimprovement in physical stability is not only due to the increasingoil viscosity but due to the viscoelastic properties of the castor oil/ethyl cellulose binary systems, which at low temperatures displaya certain elastic-like characteristics [21], similar to thoseextensively described for concentrated polymer solutions andweak gels [28]. However, above a critical temperature of around70–75 8C a Newtonian behaviour can be found for the ethylcellulose/castor oil binary systems, yielding a dramatic decrease inviscosity (see Fig. 4) and also disappearing the viscoelasticcharacteristics. Therefore, at these high temperatures, an increasein ethyl cellulose concentration is not relevant, being theconcentration of the other cellulosic derivative the only responsi-ble for the resulting bulk rheology of the oleogel. On the contrary,below ethyl cellulose sol–gel transition temperature, there is ajoint contribution of both types of thickeners. In this case,intermediates W values, in the range studied, provide oleogelrheological responses much more similar to those found instandard greases [2,5,6,22].

On the other hand, as can be deduced from Figs. 1 and 2, thevalues of the SAOS viscoelastic functions decrease as temperatureincreases in the whole frequency range studied. As can be observedin these figures, the frequency dependence of both moduli for agiven oleogel is not qualitatively influenced by temperature. Onthe contrary, the evolution of oleogel linear viscoelastic functionswith temperature largely depends on the cellulosic derivatives

0.0024 0.002 8 0.003 2 0.003 6 0.004 07

8

9

10

11

12

13

ln [G

0 N (P

a)]

1/T (K-1)

Fig. 5. Evolution of the ‘‘plateau’’ modulus (square symbols) with temperature, and

Arrhenius’ fitting (solid line), for a selected oleogel formulation (ethyl cellulose/

methyl cellulose weight ratio: 0.10).

Table 2Activation energy values, from Eq. (2), for the different oleogels studied.

W Ea (kJ/mol)

Oleogel based on ethyl cellulose

and a-cellulose

0.06 5.54

0.08 9.16

0.10 9.54

0.12 11.58

0.14 16.20

Oleogel based on ethyl cellulose

and methyl cellulose

0.04 7.39

0.06 9.69

0.08 31.09

0.10 32.58

0.12 37.85

R. Sanchez et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 705–711710

concentration ratio employed. Thus, SAOS viscoelastic functionsthermal dependence is clearly dampened by reducing ethylcellulose concentration in oleogels thickened with both a-cellulose/ethyl cellulose and ethyl cellulose/methyl celluloseblends. As previously reported for standard lithium greases[5,6], the ‘‘plateau’’ modulus can be used to quantify the influenceof temperature by using an Arrhenius-type equation:

G0N ¼ A � eðEa=RÞ�ð1=TÞ (2)

where Ea is a parameter which evaluates the thermal dependence,similar to the activation energy (J/mol), R is the gas constant

Table 3Penetration values for oleogels, model lithium soap/castor oil grease, and standard lub

Sample W Unw

(dmm

Oleogels based on ethyl cellulose and a-cellulose 0.06 335

0.08 343

0.10 380

0.12 358

0.14 354

Oleogels based on ethyl cellulose and methyl cellulose 0.04 429

0.06 440

0.08 422

0.10 410

0.12 425

COLi 242

Standard paraffinic oil-based grease 303

Standard naphthenic oil-based grease 274

(8.314 J/mol K), T is the absolute temperature (K), and A is the pre-exponential factor (Pa). Eq. (2) fits the experimental plateaumodulus values, in the whole temperature range studied, fairlywell (r2 > 0.992, see Fig. 5). Table 2 shows the activation energyvalues for all the oleogels studied. In general, Ea values increasewith ethyl cellulose/a-cellulose and ethyl cellulose/methyl cellu-lose weight ratios, although the most important changes seem tooccur when cellulosic derivatives concentration ratio, W, isincreased from 0.06 to 0.08 for both types of dispersions (Table2). Furthermore, thermal susceptibilities are generally lower foroleogels formulated with ethyl cellulose/a-cellulose blends. In thissense, Ea values found for gel-like dispersions formulated with lowcellulosic derivatives concentration ratio are only slightly higherthan those obtained for standard lithium greases in the low-temperature range [5,6]. In the case of dispersions based on ethylcellulose/a-cellulose blends, at high W values, Ea values are alsolower than those found for conventional lithium greases in thehigh temperature range (18–20 kJ/mol). On the other hand,oleogels manufactured with ethyl cellulose and methyl cellulose,using W values higher than 0.06, show much higher thermalsusceptibilities than lithium greases [6].

Finally, mechanical stability, traditionally determined forlubricating greases from the difference between penetrationindexes after and before submitting the grease to a standardizedworking test, was evaluated. In this study, oleogel mechanicalstability was estimated after the application of a shear rolling test.In general, greases with good mechanical stability must exhibitpenetration increments after working close to zero. Worked andunworked penetration values and mechanical stability data for theoleogels studied are gathered in Table 3. These values arecompared with those obtained for two standard lithium lubricat-ing greases based on both paraffinic and naphthenic oil, and amodel semi-biodegradable grease made from castor oil andlithium 12-hydroxystearate soap (CoLi). As can be observed, theconsistency (determined from penetration values) shown by theoleogels studied is generally lower than that of standard lithiumlubricating greases of NLGI grade 1–2, especially when dispersionswere formulated with ethyl cellulose/methyl cellulose blends.Therefore, these formulations could be described as soft or semi-fluid greases designated with a NLGI number 00. However, it isworth pointing out that these cellulose derivatives blends yieldoleogel formulations with appropriate mechanical stability values,similar to those found for standard greases. On the contrary, theconsistency of oleogels based on ethyl cellulose/a-cellulose blendsis more similar to that shown by most commonly employedgreases (NLGI number 0–1) but is otherwise significantly affectedby the application of a severe continuous shear, like that inducedby the standardized rolling elements.

ricating greases.

orked penetration

)

Worked penetration

(dmm)

Penetration variation

(dmm)

407 72

418 75

467 87

433 75

433 79

422 �7

440 0

425 3

410 0

429 4

260 18

305 2

289 15

R. Sanchez et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 705–711 711

4. Conclusions

The influence of cellulosic derivatives concentration ratio onthe rheological properties and mechanical stability of gel-likedispersions formulated with castor oil and ethyl cellulose/a-cellulose or ethyl cellulose/methyl cellulose blends has beeninvestigated. Changes in ethyl cellulose/a-cellulose or ethylcellulose/methyl cellulose weight ratios yield some formulationswith suitable rheological characteristics and mechanical stabilitiesfor potential lubricating applications of these oleogels. Animportant decrease in the values of the linear viscoelasticityfunctions down to a minimum value has been found by increasingethyl cellulose/a-cellulose or ethyl cellulose/methyl celluloseweight ratio (W) up to a critical value, which depends on bothnature of the cellulosic derivatives employed and temperature.Above this critical value, the viscoelastic functions increase withW, at temperatures ranging from 0 to 75 8C. However, at highertemperatures, i.e. 125 8C, a continuous decrease in the values ofSAOS viscoelastic moduli with W has been always observed, whichis related to the sol-gel transition of ethyl cellulose in castor oil.Above this transition temperature, ethyl cellulose concentration isnot relevant and the concentration of the other cellulosicderivative is the only responsible for oleogel bulk rheology.Thermal susceptibilities are generally lower for oleogels contain-ing a-cellulose in the formulation. An Arrhenius-type equation canbe used to quantify the thermal dependence of the ‘‘plateau’’modulus, which is otherwise significantly dampened by reducingethyl cellulose concentration in oleogels thickened with both ethylcellulose/a-cellulose and ethyl cellulose/methyl cellulose blends.Finally, it is worth remarking that oleogels formulated with ethylcellulose/methyl cellulose blends have shown appropriate me-chanical stabilities, which may favour their potential use as bio-lubricating greases.

Acknowledgements

This work is part of two research projects (CTQ2007–60463/PPQ and TEP-367) sponsored by a MEC-FEDER Programme and the‘‘Consejerıa de Innovacion, Ciencia y Empresa de la Junta deAndalucıa’’ (‘‘Projects of Excellence’’ Programme), respectively.

One of the authors (R. Sanchez) has received a Ph.D. Research Grantfrom the ‘‘Consejerıa de Innovacion, Ciencia y Empresa (Junta deAndalucıa)’’. The authors gratefully acknowledge their financialsupport.

References

[1] J.E. Martın-Alfonso, G. Moreno, C. Valencia, M.C. Sanchez, J.M. Franco, C. Gallegos,J. Ind. Eng. Chem. 15 (2009) 687.

[2] M.A. Delgado, C. Valencia, M.C. Sanchez, J.M. Franco, C. Gallegos, Ind. Eng. Chem.Res. 45 (2006) 1902.

[3] NLGI, Lubricating Greases Guide, National Lubricating Grease Institute, KansasCity, MO, 1994.

[4] M.S. Jhon, H.J. Choi, J. Ind. Eng. Chem. 7 (2001) 263.[5] J.M. Madiedo, J.M. Franco, C. Valencia, C. Gallegos, J. Tribol. 122 (2000) 590.[6] M.A. Delgado, C. Valencia, M.C. Sanchez, J.M. Franco, C. Gallegos, Tribol. Lett. 23

(2006) 47.[7] L. Hamnelid, in: C. Balan (Ed.), The Rheology of Lubricating Greases, ELGI,

Amsterdam, 2000.[8] M.S. Jhon, H.J. Choi, Polymer 51 (2010) 1882.[9] M.S. Jhon, S. Izumisawa, H.J. Choi, J. Ind. Eng. Chem. 9 (2003) 508.

[10] B. Wilson, Ind. Lubr. Tribol. 50 (1998) 6.[11] F.B. Flynn, NLGI Spokesman 63 (1999) 8.[12] G.R. Blackburn, NLGI Spokesman 64 (1999) 20.[13] T.E. Rajewski, J.S. Fokens, M.C. Watson, Ind. Lubr. Tribol. 52 (2000) 110.[14] E.M. Stempfel, NLGI Spokesman 62 (1998) 8.[15] A. Bhatia, R.K. Gupta, S.N. Bhattacharya, H.J. Choi, Korea-Australia Rheol. J. 19

(2007) 125.[16] S.T. Kim, J.Y. Lim, H.J. Choi, H. Hyun, J. Ind. Eng. Chem. 12 (2006) 161.[17] J.S. Choi, S.T. Lim, H.J. Choi, A.K. Mohanty, L.T. Drzal, M. Misra, A. Wibowo, J. Mater.

Sci. 39 (2004) 6631.[18] S.G. Kim, J.W. Kim, W.H. Jang, H.J. Choi, M.S. Jhon, Polymer 42 (2001) 5005.[19] C.H. Hong, K. Zhang, H.J. Choi, S.M. Yoon, J. Ind. Eng. Chem. 16 (2010) 178.[20] R. Sanchez, J.M. Franco, M.A. Delgado, C. Valencia, C. Gallegos, Green Chem. 11

(2009) 686.[21] R. Sanchez, J.M. Franco, M.A. Delgado, C. Valencia, C. Gallegos, Carbohydr. Polym.

83 (2011) 151.[22] J.E. Martın-Alfonso, C. Valencia, M.C. Sanchez, J.M. Franco, C. Gallegos, Eur. Polym.

J. 43 (2007) 139.[23] K. Almdal, J. Dyre, S. Hvidt, O. Kramer, Polym. Gels Networks 1 (1993) 5.[24] J.M. Franco, M.A. Delgado, C. Valencia, M.C. Sanchez, C. Gallegos, Chem. Eng. Sci.

60 (2005) 2409.[25] J.E. Martın-Alfonso, C. Valencia, M.C. Sanchez, J.M. Franco, C. Gallegos, Ind. Eng.

Chem. Res. 48 (2009) 4136.[26] J.A. Garcıa-Morales, J.M. Franco, C. Valencia, M.C. Sanchez, C. Gallegos, J. Ind. Eng.

Chem. 10 (2004) 368.[27] G. Moreno, C. Valencia, J.M. Franco, C. Gallegos, A. Diogo, J.C.M. Bordado, Eur.

Polym. J. 44 (2008) 2262.[28] J.D. Ferry, Viscoelastic Properties of Polymers, John Wiley & Sons, New York, 1980.[29] C. Liu, J. He, E. van Ruymbeke, R. Keunings, C. Bailly, Polymer 47 (2006) 4461.