Análisis de relaxometría magnética nuclear de la degradación molecular en lubricantes para motores de combustión interna

Received: 3 July 2020
Revised: 9 October 2020
Accepted: 17 October 2020
DOI: 10.1002/mrc.5110
RESEARCH ARTICLE
NMR relaxometry analysis of molecular degradation in
internal combustion engine lubricants
Eustaquio M. Erro |
Leandro J. Gerbino
Laboratorio de Relaxometría y Técnicas
Especiales (LaRTE), Facultad de
Matemática, Astronomía, Física y
Computación, Universidad Nacional de
Córdoba and IFEG-CONICET, Ciudad
Universitaria, Córdoba, Argentina
Correspondence
Eustaquio M. Erro and Esteban Anoardo,
Laboratorio de Relaxometría y Técnicas
Especiales (LaRTE), Facultad de
Matemática, Astronomía, Física y
Computación, Universidad Nacional de
Córdoba and IFEG-CONICET. Ciudad
Universitaria, 5000 Córdoba, Argentina.
Email: anoardo@famaf.unc.edu.ar;
emerro@famaf.unc.edu.ar
Funding information
Fondo para la Investigación Científica y
Tecnológica, Grant/Award Number:
PICT-2017-2195
| Carla C. Fraenza
| Esteban Anoardo
Abstract
A set of experimental techniques headed by proton fast field-cycling nuclear
magnetic resonance (1HFFC-NMR) were used to analyze the effects of
degradation of lubricant oil used in an internal combustion engine (ICE). Its
relaxometric, spectroscopic, and rheological properties were evaluated and
interpreted in terms of changes in the chemical structure and the involved
molecular dynamics. In order to better understand the relaxometric behavior,
chemical changes induced by heat were investigated for selected n-alkanes, as
model-systems due to their structural simplicity. Fourier transform infrared
(FTIR) spectroscopy, viscosity measurements, and foaming were used to
contrast NMR relaxometry experiments. Main observed changes associated
with oil degradation can be attributed to molecular oxidation, fragmentation,
and ramification. As an outstanding feature of this work, we show that the
relaxometric analysis can be done without any special treatment of the sample,
allowing results in less than 10 min.
KEYWORDS
degradation, lubricant, NMR, relaxometry
1 | INTRODUCTION
Internal combustion engine (ICE) lubricants are a complex mixture of hydrocarbons constituted by base oil and
specific additives.[1,2] Their main function is not only
reducing wear and friction but also providing protection
against corrosion and removing combustion residues
from the engine. A new/fresh ICE lubricant degradation
starts immediately after engine operation is set on, progressing with time according to the use and conditions of
the engine. Lubricant degradation sets in due to intrinsic
and external factors directly affecting its performance
and consequently, compromising the engine efficiency
and its useful life. This is essentially the main reason of
why condition monitoring technology for ICE lubricants
turned to be a topic of increasing interest at both
academic and industrial sectors.[3–5]
Magn Reson Chem. 2020;1–7.
Most important and commonly used methods to
sense quality and degradation of lubricant oils include
Fourier transform infrared (FTIR) spectroscopy,[6–8]
high-pressure differential scanning calorimetry,[9] bulk
oil oxidation stability,[10] cinematic viscosity,[11] total
basic and acid number, and metals by atomic absorption
spectroscopy and gas chromatography.[12] Also, nuclear
magnetic resonance (NMR) spectroscopy is often used to
analyze lubricants.[13,14]
Fast field-cycling NMR relaxometry (FFC-NMR)[15,16]
was shown to be very useful for the characterization of
lubricant oils through the involved molecular dynamics.[17] In this technique, the spin–lattice relaxation time
(T1) is measured as a function of the magnetic field
strength (B0), that is, the 1H Larmor frequency (ν0 = γ
B0/2π, where γ is the proton gyromagnetic ratio). NMR
relaxometry allows characterizing the underlying
wileyonlinelibrary.com/journal/mrc
© 2020 John Wiley & Sons, Ltd.
1
2
molecular dynamics, such as molecular rotations and
diffusion. This information is obtained through the theoretical model that is used to describe the relaxation profile.[15,16] Our physical model uses frequency domain
data from lubricant oils relaxation rate dispersions.[18]
In this work, FFC-NMR relaxometry was used to evaluate nuclear magnetic relaxation rate (R1 = 1/T1)
changes in lubricant oil samples used in a car engine.
Results are analyzed using a theoretical model that
provides us with information about rotational and translational diffusion. FTIR spectroscopy, cinematic viscosity,
and foaming test were used to explain the relationship
between molecular transformation in the lubricant oil
and relaxation rate changes. In order to better understand the FTIR and the relaxometric behavior of
hydrocarbonated molecular systems under thermal stress,
we extended the experiments to simple pure molecules
like N-octacosane, n-1-octadecene and cis-9-octadecenoic
acid. We assume that the observed thermal effects in
these molecules can be extrapolated to the molecular
blend composing the lubricant. This procedure also
serves as a further validation of the theoretical model
in use.
2 | MATERIALS AND METHODS
The commercial lubricant oil used in our experiments
was Castrol Magnatec10W-40 (semisynthetic). The aged
lubricant oil samples come from an automobile gasoline
engine. N-octacosane 99% and n-1-octadecene were
purchased from Aldrich, oleic acid was from Sintorgan,
and n-octadecane was from Shell. N-octacosane thermal
degradation was carried out by heating a sample at 250 C
for 2 h in a homemade electric-resistance thermostatic
sand bath.
Relaxometry experiments where implemented with a
Spinmaster FFC-2000 CDC Relaxometer (Stelar; Mede,
Italy) using samples of 1.5 ml. R1 was determined
using standard prepolarized (PP) and nonpolarized
(NP) sequences.[15] In all the experiments reported here,
both the polarization and acquisition magnetic fields
were set to 0.375 T (equivalent to 15 MHz for 1H Larmor
frequency). Relaxation dispersion curves for lubricant
oil samples were determined by measuring 20 points
distributed within a Larmor frequency window ranging
from 30 kHz to 15 MHz. For the rest of the samples, at
least 10 points were measured within a Larmor frequency
range from 10 kHz to 15 MHz instead. Each point was
determined through an exponential fitting of 16 acquired
single acquisitions (without phase-cycling averaging) for
different relaxation delays. The typical total time per each
relaxation dispersion curve was between 2 and 3 min. R1
ERRO ET AL.
showed to be insensitive to the time window over which
the NMR signal was acquired (after a 90 pulse). The spin
relaxation process was mono-exponential, within errors,
at all frequencies. Measurements were carried out at
30 C for lubricant oil samples and 70 C for n-octacosane
and the other studied molecules. The sample temperature
was controlled using a Stelar Variable Temperature Controller (VTC) within ±1 C. The temperature measurement was calibrated using a Cu–Al thermocouple glued
into a 10-mm NMR tube connected to a CHY 503 electronic thermometer (CHY Firemate, Tainan, Taiwan).
A Bruker IFS66VFTIR instrument, equipped with a
photoconductive MCT detector and OPUS software was
used to obtain the condensed-phase FTIR spectra. All
spectra were recorded at 3,500–400 cm−1 interval with a
resolution of 2 cm−1 from 32 coadded interferograms. A
rheometer (Physica MCR 301—Anton Paar, Germany)
was used for viscosity determinations. The cut speed
interval was 1 × 10−3 to 1 × 103 Hz, with a conic geometry (1.006 )—50 mm plate diameter, and temperature
control at 40 C with a Peltier system. For foaming
characteristics measurement, the procedure described in
ASTM D892-03 was followed. Samples maintained at a
temperature of 24 C were blown with air at a constant
rate for 5 min and then allowed to settle for 10 min. The
volumes of foam were measured at the end of both
periods.
2.1 | Theory
We analyze the relaxation data by considering translational diffusion and molecular rotations, assuming that
both contributions are statistically independent (and
dominant in different timescales)[17,18]:
Rot
R1 ðωÞ = RTr
1 ðωÞ + R1 ðωÞ,
ð1Þ
where Tr stands for the translational diffusion term and
Rot for the rotational.
For the account of translational diffusion, we consider
the force-free-hard-sphere (FFHS) model[19–21]:
RTr
1 ðω Þ =
AD
fJ ½zðωÞ + 4J ½zð2ωÞg,
Dd
ð2Þ
where
AD =
and
8 4 2 μ0 2
πγ h
n,
4π
45
ð3Þ
ERRO ET AL.
3
2
J ðz Þ =
1 + 58 z + z8
2
3
5
6
z
z
1 + z + z2 + z6 + 81
+ 648
:
ð4Þ
qffiffiffiffiffiffiffiffi
2
In these equations, ω = 2πν0, and zðωÞ 2ωd
D . D represents the relative diffusion coefficient (defined as
D ≡ D1 + D2 where the Dj are the self-diffusion coefficients of the involved molecules; d is the closest distance
between two nuclei located in different molecules, n is
the proton density, ћ is the Plank's constant divided by
2π, μ0 the vacuum magnetic permeability constant, and γ
the proton gyromagnetic ratio.
The molecular rotations contribution to the spin–
lattice relaxation rate can be expressed by a Lorentzian
form[19]:
RRot
1 ðωÞ = AR
τR
4τR
,
+
1 + ω2 τR 2 1 + 4ω2 τR 2
ð5Þ
where
AR =
3 42 μ0 2 1
γ
:
4π r 6
10
ð6Þ
With r the effective interproton distance between two
nuclei embedded in the same molecule and τR is the
rotational correlation time.
3 | R ES U L T S A N D D I S C U S S I O N
Relaxation rates dispersions and FTIR spectra obtained
for the fresh (0 km), used (10,000 km), and overused
(20,000 km) Castrol Magnatec samples are shown in
Figure 1a,b, respectively.
In Figure 1a, relaxometric profiles obtained for a
Castrol Magnatec lubricant oil with different levels of
degradation caused by an internal combustion engine are
presented as a function of the Larmor frequency. It is
clearly evidenced that the relaxation rate increases with
ageing, as observed in the whole measured frequency
range.
As can be observed in Figure 1b, six FTIR absorption
peaks (I–VI) were selected from the spectra for the evaluation of chemical changes. I (2,910 cm−1) corresponds to
C–H stretching from methyl (–CH3) and methylidene
bridge (–CH2–) groups; II (1,714 cm−1) corresponds to –
CO– stretching from carboxylic acid and aliphatic ketone
carbonyl groups; III (1,626 cm−1) corresponds to N–H
bending from amine; and IV, V, and VI (1,450, 1,368, and
723 cm−1) corresponds to C–H bending from alkane. In
concordance with relaxometric results, the FTIR spectra
show a progressive transformation of lubricant oil
F I G U R E 1 (a) R1 dispersion curves (symbols) and their
corresponding fittings (solid lines) using the model given by
Equations 1–6. Circles represent the fresh lubricant oil (0 km),
squares and triangles represent the used (10,000 km) and overused
(20,000 km) lubricant oil samples, respectively. Experiments were
realized at 30 C. (b) Fourier transform infrared (FTIR) spectra for
the fresh lubricant oil (black line), the used lubricant oil (green
line), and the overused lubricant oil (red line)
hydrocarbons molecules. The signal increase at II and III
indicates oxygen and nitrogen incorporation. A slight
increase in the peak areas ratio for I, IV, V, and VI
evidence molecular fragmentation and ramification.
Lubricant oils gradually accumulate residues from
various sources including both internal and external to
the engine. Most common are metallic particles, soot,
dust, fuel, and water, among others. In order to test the
influence of such residues in the relaxation dispersion
profile, a considerable fraction of the solid residues was
separated by centrifugation using an Allegra 21 centrifuge
(Beckman-Coulter; Brea, United States). Centrifugation
was done at 9,000 rpm during 20 min. The supernatant of
4
the centrifuged specimen was further analyzed using
the NMR relaxometry technique. Then, a comparative
analysis was carried out between the R1 dispersion curves
obtained for the centrifuged and noncentrifuged oil
(Figure 2).
As can be observed in Figure 2, both relaxation dispersion curves are indistinguishable within experimental
errors. This result means that the presence of residues
does not affect the relaxometric analysis in an appreciable way, at least for the given conditions: accumulated
residues after 20,000 km of use and centrifugation at
9,000 rpm (20 min). The fact that processes governing the
spin–lattice relaxation are hardly influenced by the
presence of solid residues represents the advantage that
used ICE lubricants can be analyzed directly, without
any special sample treatment, allowing getting results in
a few minutes.
The model given by Equation 1 was used to analyze
the data shown in Figures 1a and 2. Some of the
involved physical parameters in this model, namely, d,
D, AR (Equation 6), and τR, were calculated from data
fitting, with initial values settled within their most probable intervals (according to previous results in different
oils). The proton density n was calculated as described
in Erro et al.[18] The obtained fitting curves using Equation 1 are shown in Figures 1a and 2, where a good
agreement with data can be observed within the experimental errors. The corresponding fitting parameters are
presented in Table 1, and their numerical values are in
agreement with similar studies already reported in the
literature.[14,18,22]
F I G U R E 2 R1 dispersion curves (symbols) and their
corresponding model fittings (solid lines) using the model given by
Equations 1–6. Circles represent the overused centrifuged lubricant
oil; triangles represent the noncentrifuged overused lubricant oil,
respectively. Experiments were performed at 30 C
ERRO ET AL.
From Table 1, we learn that the relative diffusion
coefficient D shows a tendency to decrease with the
increase of use of the lubricant. This behavior can be
associated to an increase in the viscosity due to changes
in the chemical structure and oxidation of the
hydrocarbonated chains (Table 2). The values of the
closest distance between two nuclei, d, and the rotational
amplitude (AR) did not show any tendency. An increase
in the rotational correlation times τR for the fresh, used,
and overused samples was observed. This result is also
consistent with an increase in the viscosity and consequently, a slower molecular mobility (Table 2). Last
column of Table 1 shows the parameters corresponding
to the centrifuged oil R1 dispersion (Figure 2). A significant similarity with the parameters of the noncentrifuged
specimen can be observed.
In an attempt to better understand the relaxometric
changes observed in these samples, a model molecule
was chosen for its study. N-octacosane (C28H58) is a very
simple molecule and a good model to study potential
chemical modifications (ramification, fragmentation, and
oxidation). To identify molecular changes induced by
heat, a thermal degradation was carried out by heating
an n-octacosane sample at 250 C for 2 h. Figure 3a shows
spin–lattice relaxation dispersion rates obtained for
n-octacosane in its pristine state (■) and thermally
treated n-octacosane (□) at a temperature of 70 C. Figure
3b shows FTIR spectra for a nontreated n-octacosane
(black line) and thermally treated n-octacosane (red line).
Figure 3a shows that the relaxation rate values
measured for thermally treated n-octacosane (□) are
larger than those obtained for the nontreated sample
(■). In addition, it can also be observed that the
treated sample shows up a higher R1 dispersion (clearly
observable in the MHz regime). This result suggests that
the chemical modifications promoted by thermal stress
are the responsible for a more efficient spin–lattice
relaxation process.
Four FTIR regions of the spectra (I–IV) were selected
from Figure 3b for the evaluation of chemical changes in
n-octacosane molecules. I, III, and IV corresponds to
methyl (–CH3) and methylene (–CH2–) groups. The
change observed in III and IV indicates a decrease of
methylene groups, which is consistent with a fragmentation process mediated by heat. Besides, the change in the
areas ratio between I to III and I to IV, suggest that the
methyl groups population increased throughout the
thermal treatment. II corresponds to carbonyl groups
(–C O–) and only appears in the thermally treated
sample. This confirms that heat causes the oxidation of
n-octacosane. The evidence suggests that the linear
n-octacosane is oxidized, fragmented, and ramified when
is exposed to a thermal treatment of 250 C for 2 h.[23]
ERRO ET AL.
5
TABLE 1
Parameters obtained using the model given in Equation 1
Parameter
d [m] × 10
−10
Fresh 0 km
Used 10,000 km
Overused 20,000 km
Centrifuged overused 20,000 km
3.0 ± 0.1
2.9 ± 0.1
2.5 ± 0.1
2.6 ± 0.1
D [m /s] × 10−11
2.7 ± 0.6
2.1 ± 0.6
1.6 ± 0.4
1.5 ± 0.4
AR [1/s ] × 10
6.0 ± 0.4
6.0 ± 0.4
3.3 ± 0.2
3.3 ± 0.2
3.0 ± 0.2
3.9 ± 0.2
4.8 ± 0.4
4.8 ± 0.4
2
2
−9
τR [s] × 10
TABLE 2
8
Measured cinematic viscosity values
Castrol Magnatec 10W-40, T: 40 C
Use (km)
Cinematic viscosity [cSt]
(95 ± 1)a
0
10,000
(106 ± 1)
20,000
(122 ± 3)
a
Value reported by the manufacturer: 96 cSt.
F I G U R E 3 (a) R1 dispersion curves for n-octacosane in its
pristine state (■) and thermally treated state (□). Measurements
performed at 70 C. (b) FTIR spectra of fresh nondegraded
n-octacosane (black line) and thermally treated n-octacosane
(red line)
In view of our FTIR results, we can say that Castrol
Magnatec and n-octacosane exhibit the same type of
chemical alterations, which, as a whole, impact on the
relaxometric results increasing the R1 values in both
cases. Thus, R1 dispersion curves for several molecules
having different size and containing two different
functional groups were used in attempts to assess a more
precise causative significance of the observed changes in
the relaxation rates. In Figure 4, R1 dispersion curves for
three n-alkanes, n-heptane (■, C7H16), n-octadecane (□,
C18H38), and n-octacosane (C28H58) with (▲) and
without (▲) thermal treatment are presented. Oleic acid
(, cis-9-octadecenoic acid, C18H34O2) and n-1-octadecene
(♦, C18H36) R1 dispersion curves were added to Figure 4
in order to show the effect that carboxylic acid groups
and double carbon to carbon bonds have on the relaxation rate values.
The R1 dispersion curves for the n-alkanes, n-heptane
(■), n-octadecane (□), and n-octacosane (▲), presented
in Figure 4, show a regular increasing of relaxation rate
values as a function of molecular size. This can be
FIGURE 4
R1 dispersion curves for n-heptane (■), noctadecane (□), n-1-octadecene (♦), n-octacosane (▲), oleic acid
(), and thermally treated n-octacosane (▲). Experiments were
performed at 70 C
6
ERRO ET AL.
explained in terms of the molecular weight: the bigger
the molecules are, the higher the restriction in the movements they have. Such restriction plays in favor for a
more efficient relaxation process. No significant differences were observed between n-octadecane (□) and
n-1-octadecene (♦) curves, indicating that a double
carbon to carbon bond at one end of the molecule does
not affect the proton relaxation in a significant way.
Finally, the relaxation rate dispersion obtained for oleic
acid () shows that carboxylic acid groups generate a
considerable increment in relaxation rate values respect
of the obtained for n-octadecane (□). The oxygens from
carboxylic acid groups form hydrogen bond interactions
with neighboring molecules, which restricts movements
and give some “structure” to the liquid phase, doing, as
was previously mentioned, a more efficient relaxation
dynamics. Thermally treated n-octacosane (▲) shows
the higher relaxation rate values, which, based on
the evidence gathered, is caused by the chemical
restructuration produced by molecular oxidation, ramifications, and chain-rupture.
The standard test method for foaming characteristics
of lubricating oils (ASTM D892) gives us information
about how long takes certain amount of foam to disappear. Table 3 shows foam volume and permanence time
for the Castrol Magnatec samples.
The foam volume and permanence time values
depend directly from the strength and number of molecular interactions,[24] the longer time (>600 s) obtained for
the overused lubricant oil sample indicates a greater
ability to form and retain bubbles due to a mayor number
of oxidized molecules forming hydrogen bond intermolecular interactions. The oxidized molecules (which
have polar oxygenated functional groups) act as surfactants, thus reducing the surface tension and facilitating
foaming. Shorter times evidence that a smaller number of
molecules have incorporated oxygen. As a consequence,
strong intermolecular interactions like hydrogen bonding
are much less probable, thus having a minor impact
in the dynamical behavior of the lubricant molecular
blend.
TABLE 3
Foam volume and permanence time for the fresh,
used, and overused Castrol Magnatec samples
Castrol Magnatec 10W-40, T: 24 C
Use (km)
Foam volume (ml)
Permanence time (s)
0
(10 ± 1)
(150 ± 7)
10,000
(30 ± 2)
(300 ± 15)
20,000
(250 ± 20)
>600
4 | CONCLUSIONS
Results obtained in this work suggest that the progressive
increment of the relaxation rate values observed in
the fresh, used, and overused lubricant samples are
directly related to chemical transformation and structural
alterations of the molecules.
Based on our study about chemical modification in
molecules of lubricant oil and n-octacosane mediated by
heat, we are able to affirm that molecular oxidation,
ramification, and chain rupture are the main factors
affecting nuclear magnetic relaxation of protons. The
strong similitude between the overused centrifuged and
noncentrifuged lubricant oil R1 dispersions suggest that
the presence of solid combustion residuals, soot, and
metallic particles (among others), dispersed in the engine
oil, are not affecting the relaxometric behavior in an
observable way. This is a clear advantage of nuclear
magnetic relaxometry, because it allows the analysis of
lubricant oils without prior treatment of the sample.
However, this point calls for a deeper analysis because
over a threshold concentration, these components may
affect the relaxation rate dispersion. In fact, in the
present case, from Table 1, we learn that even the diffusion constant remains unaffected after removing the solid
residuals, that is, the molecular transport is not being
affected by the solid residuals accumulated in the
lubricant after 20,000 km of use.
A C KN O WL ED G EME N T S
The authors acknowledge financial and general support
from FONCYT, CONICET, and Universidad Nacional de
Córdoba. CCF acknowledges a postdoctoral fellowship
from CONICET.
DATA AVAILABILITY STATEMENT
The data that support the results here discussed are
available under request.
ORCID
Esteban Anoardo
4305
https://orcid.org/0000-0001-6667-
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How to cite this article: Erro EM, Gerbino LJ,
Fraenza CC, Anoardo E. NMR relaxometry
analysis of molecular degradation in internal
combustion engine lubricants. Magn Reson Chem.
2020;1–7. https://doi.org/10.1002/mrc.5110