CET_Analysis of the Stability of Metal Working Fluid Emulsions by Turbidity Spectra

Research Article
Analysis of the Stability of Metal Working
Fluid Emulsions by Turbidity Spectra
The physical stability of emulsions can be related to changes in the droplet size
distribution over time. Stability control of emulsions used as metal working fluids
is an important factor for the machining industry due to the decreased perfor-
mance of aged and broken emulsions. Results of turbidimetric spectra measure-
ments of metal working fluids for process control purposes and emulsion stability
monitoring are discussed. Metal working emulsions were artificially destabilized
by admixing salts which resulted in droplet coagulation. The destabilization pro-
cess was investigated by measuring the droplet size distribution and the turbidity
spectra over time. The results were evaluated based on quantitative criteria pro-
posed in the literature. The applicability of these criteria to evaluate metal work-
ing fluids during machining operations is discussed.
Keywords: Coalescence, Emulsion stability, Metal working fluids, Spectroscopy
Received: October 29, 2012; revised: April 10, 2013; accepted: April 12, 2013
DOI: 10.1002/ceat.201200590
1 Introduction
Metal working fluids (MWF) are widely used in metal process-
ing operations such as rolling, grinding, and turning, because
of enhanced process stability, work piece quality, and tool
life [1]. The consumption of metal working emulsions in
Germany amounts to about 600 000 tons of metal working
emulsions per year [2–4], while it is estimated that 2 × 109
L of
MWF emulsion is consumed worldwide, and the resulting
waste fluid generation rate may be ten times higher [5, 6].
MWF are used to decrease the thermal, chemical, and
mechanical stress in the contact zone of machining processes
which are caused by shearing and friction. Therefore, MWF
reduce the friction between the tool or abrasive particles
(chips, fines, swarfs, and residues) and the work piece. More-
over, MWF decrease the accrued heat and dissipate the pro-
duced heat caused by friction, leading to more uniform tem-
perature distribution in the machining process and an
independence of the ambient room temperature which may be
important for machining of chemically reactive alloys [7]. In
addition, MWF flush away the created fines and chips out of
the contact zone from the nascent metal surface to prevent a
rewelding and protect the newly formed surface by wetting it,
which is important for, e.g., drilling.
MWF emulsions contain mixtures of different oils and
chemical additives, e.g., emulsifiers, corrosion inhibitors, bio-
cides, and defoamers, which increase the performance of the
MWF and, therefore, of the process and product. Depending
on the machining processing operation and the work piece
material, the dispersed-phase concentration amounts to
2–10 vol % with a mean droplet size of 0.1–2.0 lm [8–10].
While more than 300 different components can take part in
the formulation of MWF emulsions, a single mixture may
contain up to 60 different components [9, 11].
MWF emulsions are mainly stabilized by adsorption of ad-
mixed emulsifiers at the liquid-liquid phase boundary due to
electrostatic and steric barrier, preventing destabilization pro-
cesses like creaming, sedimentation, flocculation/aggregation,
and coalescence as well as the complete breakage of the disper-
sion [12]. In laboratory investigations of MWF stability, two
main mechanisms are applied to artificially destabilize MWF
emulsions, namely chemical methods like the addition of salts
or acids, and physical methods like temperature increase or
application of an electric field [13, 14]. The admixed cations
reduce the surface potential of the oil droplets in accordance
to the DLVO theory since they adsorb partly at the oil surface
and lower the repulsive and electrostatic barriers at the surface
of the droplets. Furthermore, the salt increases the density of
the water which promotes the separation process [15, 16].
Thus, the stability of the emulsion decreases due to the in-
creased possibility of coalescence of the droplets and the higher
coagulation rate. This eventually leads to the complete
breakage of the emulsion. An increased temperature acceler-
ates most of the chemical reactions (like the hydrolysis of
certain emulsifiers), reduces the viscosity (by increased Brow-
www.cet-journal.com © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2013, 36, No. 7, 1202–1208
Benjamin Glasse1
Cristhiane Assenhaimer2
Roberto Guardani2
Udo Fritsching1
1
University Bremen,
Mechanical Process
Engineering Department,
Bremen, Germany.
2
University São Paulo,
Chemical Engineering
Department,
São Paulo, Brazil.
–
Correspondence: B. Glasse (glasse@iwt.uni-bremen.de), University
Bremen, Mechanical Process Engineering Department, Badgasteiner
Straße 3, 28359 Bremen, Germany.
1202 B. Glasse et al.

nian motion and coalescence rate), and changes the cloud
point which corresponds to the formation of suspended solid
particles [12, 13]. In order to adapt MWF to hard water usage
(with increased salt content), special buffer additives are ad-
mixed.
Nevertheless, due to biological, thermal, and chemical pro-
cesses in metal working machining operations, the MWF com-
position and morphology change during usage, resulting in
increased and broader droplet size distribution (DSD) which
directly affects the MWF performance and toxicity [9, 17].
Turbidity spectroscopy provides a fast technique for gather-
ing information on the chemical composition, droplet size, in-
ternal structure, and concentration of the dispersed phase of
an emulsion. Analysis of turbidity spectra may be used as a
component of optical sensors for the online and inline size
measurement of sub-lm and lm droplets due to the simple
geometrical requirements and simple light scattering method
in a relatively narrow wavelength range (usually from ultravio-
let until near-infrared range) [18–20]. The turbidity of an
emulsion is affected by the dispersed phase (particles, droplets,
and bubbles) and is related to the incident and transmitted
light for a path length using a spectrometer. The turbidity
spectrum consists of the absorption by the species present and
the scattering by the dispersed droplets, depending on the
wavelength [21]. Therefore, turbidimetric techniques may be
used to monitor the stability of MWF, since the turbidity is re-
lated to concentration as well as DSD of the dispersed phase
[14, 22, 23].
Eq. (1) relates the measured turbidity s(k0) via spectrometry
with the optical path length L, the emitted light intensity,
I0, and the received light intensity I, for light with wavelength
k0, while the term ln(I0/I) is referred to absorbance or extinc-
tion.
s…k0† ˆ
1
L
ln…
I0
I
† (1)
The absorbance of emulsions is the result of light absorption
by the continuous and dispersed phases plus scattering. For
a nonabsorbing system, the turbidity can be directly related
to scattering by the suspended droplets in the form of
Eq. (2) [21]:
s…k0† ˆ N
Z∞
0
p
4
D2
K a; m… †f …D†dD (2)
The scattering coefficient, K(a, m), depends on the droplet
size parameter a (a = pD/km) and the refractive index
ratio m, evaluated at k0. For dilute dispersions consisting of
monodisperse spherical nonabsorbing particles significantly
smaller than the wavelength of the incident light, scattering
is described by the Rayleigh scattering regime [21]. Under this
regime, and if it is assumed that the refractive index
ratio does not depend significantly on the wavelength, the
scattering coefficient, K, can be expressed in a simplified form
as Eq. (3):
K ≈ K0az
(3)
in which the pre-exponential term K0 incorporates the proper-
ties contained in the expression for the scattering coefficient
under the Rayleigh scattering regime. Under this regime, the
exponent z is equal to 4 [21] and decreases as the particle size
increases. Eqs. (2) and (3) can be combined and, for a given
particle diameter, e.g., the volumetric mean diameter, d4,3, the
wavelength exponent can be expressed as the slope of ln(s) ver-
sus ln(1/k0), as indicated in Eq. (4).
z ˆ
d ln…s†
d ln…1=k0†
(4)
where k0, wavelength in vacuum, is calculated as:
k0 = kmnm (5)
km is the wavelength and nm is the refractive index of the
continuous medium. Thus, under the mentioned assumptions,
the wavelength exponent for a given emulsion can be deter-
mined from turbidity measurements at different values of km
by fitting Eq. (4) to the data.
The simplified relationship in Eq. (4) has been the basis for
a number of studies reported in the literature [23, 24] aimed at
using the wavelength exponent as an indicator of the stability
of emulsions. Specifically in the paper by Deluhery and Raja-
gopalan [23], the decrease in the wavelength exponent over
time has been related to the destabilization of emulsions by
associating this process with the increase in droplet size by
coalescence.
In the present study, this application has been further inves-
tigated by monitoring both the turbidity spectra and the DSD
of emulsions over time, for MWF samples destabilized by add-
ing calcium chloride. For these MWF samples, the time evolu-
tion of the wavelength exponent as well as the fitting quality of
Eq. (4) to the experimental data have been measured and com-
pared with DSD measurements.
2 Materials and Methods
2.1 Materials
A commercial MWF emulsion based on mineral oil was used
without any further treatment at a concentration of 5 vol %. This
MWF type is recommended for general usage in machining
processes with hard water. Calcium chloride (CaCl2 × 2H2O,
purity of 99.5 %) was employed in the artificial MWF destabi-
lization experiments.
2.2 Laboratory Experiments
Stability experiments were carried out first with different cal-
cium chloride concentrations to visually observe the appear-
ance of haze as well as to measure changes in the DSD. A con-
centration of 0.3 wt % CaCl2 led to measurable turbidity and
stabilized DSD in an acceptable time and was used in all ex-
periments reported in this study.
Chem. Eng. Technol. 2013, 36, No. 7, 1202–1208 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
Spectroscopy 1203

2.3 Workshop Experiments
MWF emulsion samples were taken weekly from the nozzle of
a vertical turning machine. The fluid was recirculated for
5 min prior to sampling for homogenization purposes. The
concentration was approximately 5–7 vol %.
2.4 Emulsion Destabilization Experiments
The MWF was diluted with deionized water at room tempera-
ture to a concentration of 5 vol % and afterwards gently sha-
ken, creating a homogenized emulsion. The CaCl2 was mixed
with deionized water and shaken for 0.5 h on a vibrating table
until all CaCl2 was completely dissolved. Then, the salt
solution was admixed to the metal working emulsion to reach
the desired concentration of 0.3 wt % and stirred to homoge-
nize the dispersion. A series of 13 samples were prepared under
the same conditions and used in the destabilization experi-
ments.
2.5 Turbidity Spectra Measurement
Absorbance measurements were performed with a UV-Vis-
NIR spectrometer, model HR2000+ES from OceanOptics, with
a light source DH 2000-BAL (200–1100 nm) and a dip probe
with 6.35 mm diameter, 127 mm length, and 2 mm optical
path length which enables inline monitoring of the MWF
emulsion. The noise and reference signals were recorded prior
to the measurements and subtracted from the measurement
signal with an integration time of 10 ms and averaged for ten
measurements in the ultraviolet to near-infrared light range
(215–1050 nm). The light source was initially warmed up for
45 min in order to reach its full intensity and the sample was
gently shaken prior to the measurements, in order to homoge-
nize it.
2.6 Droplet Size Measurement
The volumetric DSDs of the MWF emulsions were measured
by laser diffraction with a Malvern Mastersizer 2000 diffrac-
tometer, applying an independent model and refractive index
of the MWF equal to 1.45. It was necessary to dilute the sam-
ples by adding water in order to prevent multiple scattering.
No destabilization was observed after water addition, since the
DSD of the diluted samples did not change after 1 h.
3 Results and Discussion
After addition of the CaCl2 solution, the MWF samples be-
came immediately cloudy. As illustrated in Fig. 1, the absor-
bance measured by the spectrometer increased over the whole
spectra. Changes in shape and an increase in the oscillations of
the turbidity curves are also demonstrated in Fig. 1, indicating
that the turbidity spectra are very sensitive to the destabiliza-
tion caused by adding the CaCl2 solution to the MWF. In a
previous study by the authors [25], it was observed that no
constituent of this MWF absorbs light in the range from 400
to 900 nm. Thus, the observed changes in the spectra in this
range over time are mainly due to changes in the droplet
population. In this study, the results are based on the absor-
bance measured in the range from 500 to 605 nm, in order to
avoid any oscillation in the spectra that could be related to
light absorption effects.
Fig. 2 presents results obtained with an MWF sample at two
different times after addition of CaCl2 solution. The observed
changes in the absorbance spectra (plot (a)) correspond to a
significant decrease in the slope of the straight lines (plot (b)),
i.e., the wavelength exponent z obtained by linear regression of
the data based on Eq. 4. Plot (c) indicates that this destabilized
MWF contains two droplet populations.
As illustrated in Fig. 2, the addition of CaCl2 resulted in sub-
stantial changes in the DSD of the MWF, with the formation
of a second droplet population with larger diameters. The
droplet size distribution was monitored over time for the arti-
ficially destabilized MWF samples and for the MWF used in
the turning machine. The results are displayed in Fig. 3. The
DSD changes gradually from monomodal to bimodal. The
larger mode corresponds to the new population formed. This
larger mode gradually shifts towards larger droplet sizes and
the DSD curve becomes progressively broader. Fig. 3 b shows a
similar behavior of the DSD of the MWF used in the turning
machine over time.
Figs. 4–6 present results for the artificially destabilized MWF
samples after CaCl2 addition. The volumetric mean droplet
diameter, d4,3, increased over time from approx. 150 nm to
700–1700 nm. This behavior was expected, since the addition
of salts to an emulsion changes the electrostatic balance of the
system, decreasing the repulsive forces between the droplets
and facilitating the coalescence process. However, the DSD ap-
parently tends to stabilize after ca. 1000 min. The dispersion of
the DSD curves also increases with time as a consequence of
the formation of the bimodal distribution and tends to stabi-
lize for longer times. The corresponding values of the wave-
length exponent, z, are indicated in Fig. 6. These values were
Figure 1. Absorbance spectra of an MWF sample at different
times after addition of 0.3 wt % CaCl2.

estimated by linear regression from turbidity wavelength data
based on Eq. (4).
The decrease of the z-values is in accordance with the results
reported by Deluhery and Rajagopalan [23] and also with the
predicted tendency from scattering equations [21]. However,
the formation of a bimodal DSD results in a significant de-
crease in the quality of the fitting of Eq. (4) to the data. This is
illustrated in Fig. 7 for the artificially destabilized samples. As
expected, the wavelength exponent decreases gradually with
the increase in d4,3, but there is a significant reduction in the
quality of the fitting as expressed by the coefficient of determi-
nation, R2
, when the volumetric mean diameter, d4,3, reaches
ca. 10 lm.
Figure 2. Experimental results with an MWF sample at two dif-
ferent times after addition of 0.3 wt % CaCl2. (a) Absorbance
spectra; (b) ln(s) versus ln(1/km) (Eq. (4)); (c) DSD.
Figure 3. (a) DSD of the MWF samples at different times after
addition of CaCl2; (b) weekly change of the DSD of a used MWF
before refilling of the machine.
Figure 4. Time evolution of the volumetric mean (d4,3) of the
DSD for the MWF samples after addition of 0.3 wt % CaCl2.
Spectroscopy 1205

The use of the wavelength exponent has been proposed un-
der the assumption of a monomodal and monodisperse distri-
bution [23], and the decrease in its value with time has been
associated to the growth in droplet size by coalescence. Thus,
according to [23], the stability of an emulsion can be evaluated
by measuring the turbidity at different wavelengths over a cer-
tain time period and monitoring the time evolution of the
wavelength exponent obtained by fitting Eq. (4) to the data.
However, based on the results in Fig. 7, the fitting quality of
Eq. (4), e.g., the coefficient of determination, and the resulting
wavelength exponent are measured at specific instants of time,
and then the condition of the MWF emulsion can be evaluated
in real time.
This method was applied to the monitoring of an MWF
under real operation conditions, i.e., in a turning machine.
The results are illustrated in Fig. 8 for an operating time of
33 weeks without replacement of the MWF. The wavelength
exponent decreases over time and, after ca. 28 weeks, a sharp
drop in the coefficient of determination is observed.
In Fig. 9, the condition of the MWF is indicated at different
operating times of the machine. In the first week of operation,
the emulsion contains essentially one population of droplets.
As a result, the fitting of Eq. (4) is good (R2
equal to 0.997).
For 30 weeks of operation, the DSD is multimodal, and the
fitting of Eq. (4) is significantly worse (R2
equal to 0.481).
4 Conclusions
The destabilization process of a commercial MWF emulsion
with MWF concentration of 5 vol % artificially aged with
CaCl2 was characterized by UV-Vis-NIR spectroscopy. Shortly
after addition of this salt to the MWF, the DSD of the emul-
sion became bimodal. A new population with larger droplets
increased in volume and in mean droplet size over time, even-
tually reaching a stable condition, characterized by the pres-
ence of two droplet populations.
Monitoring of the MWF destabilization process by the spec-
trometric turbidity sensor and the use of the turbidity spectra
to calculate the wavelength exponent pointed to a clear corre-
lation with the volumetric mean droplet size.
The results confirmed the conclusions previously presented
by other authors [23] that the wavelength exponent can serve
as a quantitative criterion in the monitoring of the emulsion
destabilization process by observing the change in its value
Figure 5. Time evolution of the standard deviation of the DSD
for the MWF samples after addition of 0.3 wt % CaCl2.
Figure 6. Time evolution of the wavelength exponent z (Eq. (4))
for the MWF samples after addition of 0.3 wt % CaCl2.
Figure 7. Wavelength exponent z and coefficient of determina-
tion R2
for the fitting of Eq. (4) to data of the artificially destabi-
lized MWF samples as a function of d4,3.

over a given time period. Furthermore, in the present study,
the time evolution of the emulsion structure was further inves-
tigated and the detected decrease in the wavelength exponent
was associated with the formation of a second droplet popula-
tion with larger mode. As the volume fraction of this second
population with larger mode increases, the quality of the fit-
ting of Eq. (4), expressed by its coefficient of determination,
decreases. Thus, the emulsion stability can be evaluated by per-
forming one instantaneous measurement of turbidity at differ-
ent wavelengths, with computation of the coefficient of deter-
mination of the wavelength exponent fitting.
In view of the simple measuring and computation method,
this concept of applying the wavelength exponent was tested
in the operation of a turning machine during approx.
30 weeks. The results indicate that the method can be applied
to the in situ characterization of MWF quality in machining
processes.
Acknowledgment
The authors would like to thank the Deutsche Forschungs-
gemeinschaft (DFG) and the Brazilian partners Coordenação
de Aperfeiçoamento de Pessoal de Nível Superior (CAPES),
Conselho Nacional de Desenvolvimento Científico e Tecnoló-
gico (CNPq), Fundação de Amparo à Pesquisa do Estado de
São Paulo (FAPESP), and Financiadora de Estudos e Projetos
(FINEP) who funded this project within the Brazilian German
Collaborative Research Initiative in Manufacturing Technology
(BRAGECRIM). The authors would like to thank the Manu-
facturing Department of the Foundation Institute of Materials
Science Bremen for supporting this project.
The authors have declared no conflict of interest.
Symbols used
d [lm] droplet size
f(D) [lm–1
] droplet size distribution density
function
I [cd] received light intensity
I0 [cd] emitted light intensity
K [–] scattering coefficient in Eq. (1)
K0 [–] pre-exponential term in Eq. (3)
L [cm] optical path length
N [cm–3
] number of particles per unit
emulsion volume
Figure 8. Time change of the wavelength exponent z and the
coefficient of determination R2
for the fitting of Eq. (4) to MWF
samples taken from a turning machine in operation. Figure 9. (a) Illustration of the DSD and (b) fitting of Eq. (4) to
MWF samples taken from a turning machine operation at differ-
ent operating times.
Spectroscopy 1207

m [–] refractive index ratio np/nm
nm [–] refractive index of the
continuous phase
np [–] refractive index of the droplets
R2
[–] coefficient of determination
z [–] wavelength exponent
Greek symbols
a [–] droplet size parameter pD/km
k0 [nm] light wavelength in vacuum
km [nm] light wavelength in the medium
k0/nm
s [a.u.] absorbance, turbidity
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