Turbidimetry for the Stability Evaluation of Emulsions Used in

Machining Industry

Benjamin Glasse,1* Cristhiane Assenhaimer,2 Roberto Guardani2 and Udo Fritsching3

1. Particles and Process Engineering, University Bremen, Badgasteiner Straße 3 28359, Bremen, Germany

2. Chemical Engineering Department, University São Paulo, São Paulo, Brazil

3. Particles and Process Engineering, University Bremen, Bremen, Germany

Emulsified fluids are used inmany industrial and consumer areas, for instance as products in the food or health industry aswell as technical fluids in themachining industry. Metalworking fluids (MWF) are used as coolants and lubricants in metalworking processes. During their usage MWF emulsionsmay change their physical and chemical properties, which influences their performance anddecrease the physical stability and therefore their lifetime.This article discusses results of turbidimetric spectra measurement of MWF emulsions to be used for process control, MWF quality monitoring andformulation purposes. Therefore, laboratory experiments have been carried out investigating the physical stability. Metal working emulsions havebeen treated and destabilised by different concentrations of salts. The destabilisation process was monitored by undiluted turbidity measurementsand evaluated by the temporal change of the wavelength exponent. Thus, it was possible to determine specific conditions, for example a specificcritical salt concentration for maintaining, stability of the MWF formulations.

Keywords: metal working fluid, emulsion stability, turbidity analysis

INTRODUCTION

Metal Working Fluids

Metal working fluids (MWF) are used in metal processingoperations such as rolling, grinding and turning, aswell asfor enhanced manufacturing process stability, work piece

quality and increased tool life.[1]MostMWFare formulated as oil‐in‐water (O/W) emulsions.[2–4] The use ofMWFdecreases the thermal,chemical andmechanical stresses caused by shearing and friction inthe contact zone of the tool and the work piece of the machiningprocesses as well as flushes away the created fines and chips fromthe nascent metal surface, thus preventing rewelding and providingprotection for the newly formed surface by wetting it.[3]

Depending on the machining processing operation and the workpiece material, the disperse phase concentration in aMWF is in theorder of 2–10% v/v with a mean droplet size of 0.1–2.0mm.[5,6]

MWF emulsions contain mixtures of different oils (mainly mineraloils) and chemical additives, for example emulsifiers, corrosioninhibitors, biocides and defoamers, which increase the perfor-mance of the MWF. More than 300 different components may beused inMWF formulations, where a singlemixturemay contain upto 60 different components.[5,7] So‐called ‘green’ or biodegradableoils recently are gaining more interest for the formulation of MWF.The participation of biodegradable oils or additives in metalworking processes has increased in recent years due to anincreased regulation of industry contamination and pollutionand an increased awareness of the public, leading to an increasedenvironmental friendly production.[8,9]

MWF are mainly stabilised by adsorption of amphiphilic surfaceactive molecules (emulsifiers at the liquid‐liquid phase boundarydue to electrostatic (ionic emulsifiers) and steric (non‐ionicemulsifiers) barrier), preventing destabilisation processes likecreaming, sedimentation, flocculation/aggregation and coales-cence, that can lead to the complete phase separation of the waterand oil phase.[10] Due to biological, thermal and chemical

processes in metal working machining operations, the MWFcomposition and morphology may change during usage, resultingin an increased and broader droplet size distribution (DSD), whichdirectly affects the MWF performance.[5,11] For analysing thebehaviour and stability of MWF‐emulsions under laboratoryconditions, the emulsions may be artificially aged. Two mainmechanisms may be used to destabilise a MWF emulsion in acontrolled way: chemical methods like the addition of salt oracids,[12] and physical methods, like an increased temperature oran electric field.[3] According to the DLVO theory, admixed cationsreduce the surface potential of the oil droplets since they adsorbpartly at the oil surface and lower the repulsive and electrostaticbarriers at the surface of the droplets. An increased temperaturecan accelerate the destabilisation by decreasing the viscositycausing an increase in Brownian motion and coalescence rate, andchanging the cloud point, where some dissolved solids are nolonger completely dissolved.[3,10]

Turbidity Spectroscopy

Turbidity spectroscopy (also referred to as turbidimetry or UV‐Visspectroscopy) is an easily applied light scattering measurementtechnique,[13–15] providing information about the chemical com-position, droplet size, internal structure and concentration of thedispersed phase of an emulsion.[16,17] Themeasured turbidity t(l0)is given by the path length L, the emitted light intensity I0 and theattenuated light intensity I for wavelength l0 in vacuum as:

*Author to whom correspondence may be addressed.E‐mail address: glasse@iwt.uni-bremen.deCan. J. Chem. Eng. 92:324–329, 2014© 2013 Canadian Society for Chemical EngineeringDOI 10.1002/cjce.21930Published online 31 October 2013 in Wiley Online Library(wileyonlinelibrary.com).

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tðl0Þ ¼ 1Lln

I0I

� �ð1Þ

where the term ln(I0/I) is the absorbance or optical density. Thewavelength l0 is calculated by:

lm ¼ l0

nmð2Þ

where lm is the wavelength in the medium and nm the refractiveindex of the continuous medium at wavelength lm. The turbidityspectrum consists of the absorption of the materials and thescattering of the dispersed droplets over a wavelength range.[18]

Therefore, turbidimetric techniques may be used to follow thedestabilisation process of MWF, since the turbidity is related toconcentration as well as to the particle size of the dispersedmedium.[12,19–21]

Wavelength Exponent

An analytical method for the evaluation of turbidity spectra is thewavelength exponent z, defined by:

z ¼ d lnðtðl0ÞÞd lnð1=lmÞ ð3Þ

calculated from a linear regression applied to the whole measuredturbidity spectrum. The derivation of the wavelength exponentfrom the turbidity spectra (Figure 1a) of two mono‐modal log‐normal droplet size distributions (DSD) is illustrated in Figure 1b,where the wavelength exponent amounts 2.84 for a mean dropletsize of 200nm and 2.25 for a mean droplet size of 500nm.According to Deluhery and Rajagopalan[19] the turbidity of twowidely separated wavelengths leads to the wavelength exponent zfor spherical mono‐disperse particles, which are independent fromthe concentration of the dispersed phase, if multiple scattering isavoided. A temporal change of the wavelength exponent indicatesa change of the droplet size.[14,19]

Figure 2 illustrates the dependency of the wavelength exponentversus the mean droplet size DG of a simulated mono‐modal log‐normal droplet size distribution in the size range of 50–40 000nmwith differing standard deviations s (0.1� s� 0.5), where theinner figure illustrates the principle influence of the broadness s onthe size distribution. The simulations have been carried out with anumerical code, where the calculation of the Mie theory[22] is donewith MATLAB codes based on Bohren and Huffmann.[18]

The wavelength exponent decreases to a global minimum fordroplet sizes of 2 000–3 000nm, and oscillates around value of zerofor increasing droplet sizes where the amplitude of this oscillationdecreases for increasing droplet sizes. A decrease of thewavelengthexponent indicates an increase of the droplet size for submicronMWF emulsions, due to the non‐objectivity since no wavelengthexponent could be assigned to one specific droplet size for micronemulsions. The oscillation of the wavelength exponent around avalue of zero vanishes for droplet sizes >10mm due to scatteringattributes of the droplets, a broader droplet size distributionincreases this cushioning due to an overlapping of the localmaxima and minima of the droplets.

MATERIALS AND METHODS

Materials

Different salts were used to destabilise the MWF under laboratoryconditions. Technical pure aluminium chloride, magnesium

Figure 1. (a) Simulated turbidity spectra for two log‐normal mono‐modaldroplet size distributions with a mean droplet size of 200 and 500nm.(b) Determination of the wavelength exponent with a linear regression fitfrom the turbidity spectra.

Figure 2. Dependence of the wavelength exponent from themean dropletsize of a mono‐modal log‐normal size distribution with differingbroadnesses of the distribution.

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chloride and calcium chloride have been applied without furtherpurification. Two different MWF‐formulations from differentsuppliers have been tested (MWF1 and MWF2) that are commer-cial available ‘green’ MWF emulsions, since their formulation isbased on renewable materials.

Methods

Stable oil‐in‐water emulsions have been prepared by adding MWFsamples to deionised water in the range of 0.4–22.9mass%. Thedifferent salts were dissolved in deionised water in order toproduce ionic solutions at a concentration of 10mass%. The MWFsamples and the salt solutionwere separately shaken on a vibratingtable for several minutes, creating a stable oil‐in‐water emulsionand ionic solution. Afterwards the ionic solution was graduallyadded to the MWF emulsion until the desired concentration wasreached and the system was homogenised.

The turbidity of different samples was measured in the visible tonear infra‐red light range of 400–1100nm with a HR2000þspectrometer which was connected via a dip probe (300mm fibreand 2mm path length) with a DH2000BAL light source (all OceanOptics). The dark noise and reference spectra (deionised water)were recorded prior to the measurements. The spectra wereobtained with an integration time of 10ms and averaged over 10different measurements. The wavelength exponent was calculatedby Equation 3 for all recorded measurements in the range of 600–700 nm with a spectral resolution of Dl¼ 0.47 nm and fitted to alinear regression.

RESULTS AND DISCUSION

Concentration

Figure 3a illustrates the measured wavelength exponent of MWF1for different MWF concentrations, whereas Figure 3b illustratesthe corresponding turbidity spectra. The wavelength exponent—concentration curve may be divided into four regions. The firstconcentration region of about 0–2mass% is characterised by a flatshape of the absorbance spectra and thus high wavelengthexponents, which surpass even the calculated maximal wavelengthexponent of four for synthetic data shown in Figure 2; thisconcentration marks the lower boundary for this MWF emulsion,due to the dependency of the wavelength exponent fromthe concentration. The second region of 2–15mass% showscontinuously decreasing absorbance for increasing wavelengths ofthe light in the absorbance spectra and an independency of thewavelength exponent from the concentration; therefore, this is therange of application for this measurement technique which alsomarks the conventional range of concentrations of MWF emulsions.A saturation in the absorbance readings at low wavelengths(l¼ 400–450nm) is observed when the MWF concentration isfurther increased. This saturation extendsover thewholewavelengthrange for further increasing of the concentration (l¼ 400–550nm),which is characterised by the third region for a concentration ofapproximately 15–17mass%. Hence, the third region marks theupper boundary of application of the technique due to thedependency of the wavelength exponent from the concentration, afurther increased concentration of>17mass% leads to awavelengthexponent of about zero due to the noisy flat shape of the absorbancespectra, because of the high turbidity of the dispersion in this region.

The dependency of the wavelength exponent from the absor-bance at a single wavelength of 650nm can be divided into threeregions as shown in Figure 4. The first and the third region show adependency of the wavelength exponent form the absorbance. Thewavelength exponent is independent from the absorbance in

the second region, where an absorbance >0.3 indicates multiplescattering.[23,24] Thus, the wavelength exponent may be appliedfor concentrations with occurring multiple scattering. Thecorresponding mono‐modal narrow droplet size distribution of

Figure 3. (a) Dependence of the wavelength exponent for differentconcentrations of MWF1. (b) Turbidity spectra for different concentrationsof MWF1.

Figure 4. Dependence of the wavelength exponent from a singleabsorbance at a wavelength of 650nm.

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the freshMWFemulsionwith amean droplet size of approximatelyd4,3� 150nm (measured via laser diffraction) is not affected by theconcentration for short time periods, but may lead to a decreasedlong‐term physical stability due to the increased coagulationprobability of the droplets.

Destabilisation

A change in the wavelength exponent may indicate a change in theemulsion droplet size. In this study ionic solutions were used todecrease the physical stability of MWF emulsions. However, sincecommercial products have been used, admixed hard water agentsto the MWF may provide some tolerance with respect to the salts.

Turbidity spectra of sample MWF1 with different salt concen-trations are shown in Figure 5 for a) CaCl2, b) Al2Cl3 and c) MgCl2.A change of the wavelength exponent may be used as an indicatorof the physical stability of aMWF formulation. For a stable dispersesystem, the wavelength exponent does not change over time. Forthe conditions adopted in the present study, the plots in Figure 5show that the wavelength exponent tend to decrease over time forsalt concentrations above 7mass% for CaCl2, 3mass% for Al2Cl3,and 6mass% for MgCl2 (related to the amount of MWFconcentrate). The change of the wavelength exponent for MWF1with 3.5mass% of Al2Cl3 takes place at about 6000 seconds,whereas 3.7mass% salt addition leads to a practically instanta-neous drop. Therefore, the observation time is an important factorin this method and the proposed time of 10min by Deluhery andRajagopalan[19] may be too short in cases such as those shownhere, where changes of the wavelength exponent were observedafter 10min (for wavelength exponents >0), for example MWF1with MgCl2 or MWF2 with Al2Cl3. An increase of admixed ionconcentration leads to an instant decrease of the wavelengthexponent due to the rapid increase in turbidity. This effect has alsobeen observed by Deluhery and Rajagopalan[19].

Figure 6 shows images illustrating the phase separation ofMWF1 with admixed Al2Cl3 ions, where the amount of added saltincreases from left to the right sample. The first picture on the leftshows the freshly created MWF emulsion and the last sample(right) shows the broken emulsion after 12 h. The phase boundaryrises with increasing ion amount due to the density differencebetween the aqueous and the oily phase, leading to coagulation andcreaming, and finally to the complete phase separation.

Figure 7 plots the results for an increased destabilisation ofMWF2 for a) CaCl2, b) Al2Cl3 and c)MgCl2,whichwas observed forsalt concentrations above the following values: CaCl2 5mass%,Al2Cl3 3mass% and MgCl2 6mass%.

The turbidity spectra of a MWF2 sample with 8.0mass% CaCl2as well as the corresponding calculated wavelength exponents areshown in Figure 8. The turbidity spectra increase over the wholewavelength range due to an increase of the droplet size as shownbefore. Anyway, an increased noise was observed in the near‐UV

Figure 5. (a) Determination of the physical stability of MWF1 for CaCl2.(b) Determination of the physical stability of MWF1 for Al2Cl3.(c) Determination of the physical stability of MWF1 for MgCl2.

Figure 6. Bottle test of MWF1 for increasing amount (from left to rightincreasing) of admixed CaCl2 solution after 12h, left: fresh emulsion andright: highest CaCl2 concentration.

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light rangewhich leads to a vanishing of the signal for wavelengthssmaller than 550 nm after 15min, which ‘spreads’ towards higherwavelengths. This behaviour was also observed for the near‐IRlight range after 30min, which ‘spreads’ towards lower wave-lengths. The measurement disturbances have been also observed

in the corresponding values of the wavelength exponents, whichdrop towards nearly �40. Thus, a noisy turbidity signal leads toproblems in the calculation of the wavelength exponent.

SUMMARY AND CONCLUSIONS

The use of turbidimetry for the characterisation of the stability ofcommercially available bio‐based ‘green’ metal working fluid(MWF) emulsions has been demonstrated. In laboratory experi-ments MWF have been destabilised by salts (Al2Cl3, CaCl2 andMgCl2). This aging effect can also occur during machiningprocesses, caused by accumulation of ionic solutes. The proposedsetup may be used for the rapid evaluation of different metalworking fluid formulations by the identification of a critical saltconcentration or for the online evaluation of the physical stabilityof the emulsion, due to the dependency of thewavelength exponentin relation to the droplet size of the dispersed oleos phase. Thelimits of the usability of this method have been identified by Mie‐simulations of the wavelength exponent for different droplet sizesas well as in concentration experiments.

ACKNOWLEDGEMENTS

The authors thank the Deutsche Forschungsgemeinschaft (DFG)and the Brazilian partners Coordenação de Aperfeiçoamento de

Figure 7. (a) Determination of the physical stability of MWF2 for CaCl2.(b) Determination of the physical stability of MWF2 for Al2Cl3.(c) Determination of the physical stability of MWF2 for MgCl2.

Figure 8. (a) Absorbance spectra of MWF2 for 8.0 m% CaCl2 for differentmeasurement times after the addition. (b) Calculatedwavelength exponentof MWF2 for 8.0 m% CaCl2 for different measurement times after theaddition.

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Pessoal de Nível Superior (CAPES), Conselho Nacional deDesenvolvimento Científico e Tecnológico (CNPq), Fundação deAmparo à Pesquisa do Estado de São Paulo (FAPESP) andFinanciadora de Estudos e Projetos (FINEP) who support thisproject Emulsion Process Monitor (EPM) in metal workingprocesses within the Brazilian German Collaborative ResearchInitiative in Manufacturing Technology (BRAGECRIM) at theUniversity of Bremen and University of São Paulo.

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Manuscript received December 28, 2012; revised manuscriptreceived May 21, 2013; accepted for publication June 09, 2013.

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