on-site additive depletion and oxidation monitoring in ... · turbine oil beer’s law (absorbance...
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On-site Additive Depletion and Oxidation Monitoring in Turbine Oils by FTIR Spectroscopy
By Christopher SassoOut of Lab Product SpecialistAgilent Technologies
FT-IR Spectroscopy Fundamentals
Overview of Antioxidants and Oxidation
Measuring Phenolic and Aminic Antioxidants in Turbine Oil
Relationship Between AO Depletion and Oxidation
Remaining Useful Life Results• Portable FT-IR • Other Methods
Contaminant Detection
Portable FT-IR Spectrometers
• “Spectroscopy” is the study of how electromagnetic radiation interacts with the atoms and molecules that compose matter
• “Infrared” spectroscopy is specifically the study of how infrared light (heat) is absorbed by the bonds between atoms that form molecules
What is Spectroscopy?
Microwave
(Rotation)
Radio Wave
(Nuclear Spin)
Visible
Infrared
(Molecule Vibration)
Wavelengths:
770 nm – 50 µm
Wavenumbers:
12,900 – 200 cm-1Ultraviolet
(Electron States)
X-ray
(Nuclei)
n Wavelengths of light in the infrared (heat) region have energies that are on the same order of magnitude as vibrating bonds in molecules
Infrared Radiation
Peaks appear where sampleabsorbs light
Infrared(heat)
Source
InfraredDetector
A spectrum is a graph of how much infrared light is absorbed by molecules at each wavenumber of infrared light
Infrared Spectrum
Bend
~ 1600 cm-1
Symmetric stretch
~ 3300 cm-1
Antisymmetric stretch
~ 3400 cm-1
Ab
sorb
ance
Wavenumber (cm-1)
Oxygen
Hydrogen
Infrared Spectrum of Water (H2O)
Simple IR Group Frequency Chart
H – C – C – C – C – C – C – H
H|
H|
H|
H|
H|
H|
|
H
|
H
|
H
|
H
|
H
|
HC – H stretch
H – C – H bend
Ab
sorb
ance
Wavenumber (cm-1)
Hexane
H – C – C – C – C – C – C – O – H
H|
H|
H|
H|
H|
H|
|
H
|
H
|
H
|
H
|
H
|
H
C – O stretchO – H stretch
Ab
sorb
ance
Wavenumber (cm-1)
Hexanol
H – C – C – C – C – C – C – H
H|
H|
H|
H|
H|
O||
|
H
|
H
|
H
|
H
|
HC = O stretch
Ab
sorb
ance
Wavenumber (cm-1)
Hexanal
H – C – C – C – C – C – C – O – H
H|
H|
H|
H|
H|
O||
|
H
|
H
|
H
|
H
|
H
C – O stretch
C = O stretch
O – H stretch
Ab
so
rban
ce
Wavenumber (cm-1)
Hexanoic acid
Ephedrine
Pseudoephedrine
Specificity of Infrared Spectra
IR Spectral Overlay of an Additive in an Aromatic Solvent
4600 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Wavenumber
Ab
sorb
ance
1710 1700 1690 1680 1670 1660 1650 1640
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Wavenumber
Abs
orba
nce
Calibration Curve for Additive in Aromatic Solvent
1
2
3
4
5
6
7
8
R²=1.000
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
3.0
1.5
0.0
Peak Area
Con
cent
ratio
n
Overview of Antioxidants
Aminic and Phenolic Based Additives
Sulfur based, ZDDP
Function as preservatives
• Prevent oxidation• Inhibit the oxidation mechanism
Will not reverse oxidation damage
Once consumed oxidation skyrockets
Antioxidants
Phenolic Antioxidants (AO)
• Common in most mineral based lube oils• Similar in function to natural antioxidants such as vitamin E,
(tocopherols)• Efficient free radical scavengers• Terminate the oxidation chain reaction• Sacrificial additive, first to be consumed• Better performance at low temperatures
R
OH
Antioxidants
Aminic Antioxidants
• Also common in most mineral based lube oils• Functions similarly to phenolics, quenches oxidation free radicals• Synergistic Relationship with phenolic AO
– Can be recharged by phenolic AO• High thermal stability• Performs better than phenolics at high temperatures
NH
R R
Oxidation
Conditions that lead to oil oxidation• Heat• Extreme Pressures• High sheer conditions• Water
– Accelerates oxidation by forming peroxides– Can cause wash out of antioxidants and other additives
• Metal wear particles– Acts as a catalysts that accelerate oxidation
• Electrostatic sparking – Introduces nitrates
Oxidation
The Oxidation Chain Reaction• Held in check by AO• Skyrockets without AO
– Exponential growth
– Hockey stick model
– Reaches a critical saturation point before taking off
Point of no return• Adding AO after critical saturation point has little benefit
Varnish consists of mostly oxidation products• Observed increase in varnish as AO’s are consumed
Measuring Phenolic and Aminic Antioxidants in Turbine Oil
IR bands are distinctive for aminic and phenolic functional groups
Phenolic functional group creates a sharp band at 3650cm-1
Aminic functional group is at 3430cm-1 (broad), and sharp p-aromatic at 1510cm-1
Multiple bands in the IR spectra for both AO
Measuring Phenolic and Aminic Antioxidants in Turbine Oil
3680 3675 3670 3665 3660 3655 3650 3645 3640 3635 3630 3625 3620 3615 3610Wavenumber
Abs
orba
nce
3680 3675 3670 3665 3660 3655 3650 3645 3640 3635 3630 3625 3620 3615 3610Wavenumber
Abs
orba
nce
3460 3455 3450 3445 3440 3435 3430 3425 3420 3415 3410 3405 3400 3395Wavenumber
Abs
orba
nce
3460 3455 3450 3445 3440 3435 3430 3425 3420 3415 3410 3405 3400 3395Wavenumber
Abs
orba
nce
R
OH
R
OH
NH
R R
NH
R R
Aminic aDPA, alkyl di-phenylaminePhenolic DBPC, di-tertiary-butyl paracresol
Infrared Spectrum of Turbine Oil
433
2.0
4259
.9
4168
.7
406
8.7
365
2.6
360
9.7
348
7.5
336
3.0
2727
.326
75.
1
2406
.123
39.2
2185
.2
203
1.5
189
0.8
160
6.1 15
16.1
130
5.5
125
1.5
114
4.4
112
3.1
106
7.3
937
.489
1.1
846
.0
770
.47
23.1
4600 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Wavenumber
Abs
orb
ance
3993.2 am 5995 phen w surf(1)
4332
.0
4259
.9
4168
.7
4068
.7
3652
.6
3609
.7 3487
.5
3363
.0
2727
.3
2675
.1
2406
.1
2339
.2
2185
.2
2031
.5
1890
.8
1606
.1
1516
.1
1305
.5
1251
.5
1144
.4
1123
.1
1067
.3
937.
4
891.
1
846.
0
770.
4
723.
1
3750 3700 3650 3600 3550 3500 3450 3400 3350 3300 3250 3200
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
-0.04
-0.05
-0.06
-0.07
Wavenumber
Abs
orba
nce
3993.2 am 5995 phen w surf(1)
4332
.0
4259
.9
4168
.7
4068
.7
3652
.6
3609
.7
3487
.5
3363
.0
2727
.3
2675
.1
2406
.1
2339
.2
2185
.2
2031
.5
1890
.8
1606
.1
1516
.1
1305
.5
1251
.5
1144
.4
1123
.1
1067
.3
937.
4
891.
1
846.
0 770.
4
723.
1
1670 1660 1650 1640 1630 1620 1610 1600 1590 1580 1570 1560 1550 1540 1530 1520 1510 1500 1490
0.26
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
-0.06
Wavenumber
Abs
orba
nce
3700 3690 3680 3670 3660 3650 3640 3630 3620 3610
Wavenumber
Abs
orb
anc
e
Measuring Phenolic and Aminic Antioxidants in Turbine Oil
Peak Area
1
4
7
10
13
16
19
22
25
28
Quant Validation Plot for Phenolic (ppm)R²=1.000
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
Con
cent
ratio
n
Measuring Phenolic and Aminic Antioxidants in Turbine Oil
Beer’s Law (Absorbance Law): IR absorbance is directly proportional to concentration
A=abc
A is Absorbance, b=thickness, c=concentration, a=absorptivity constant
FT-IR measures each AO separately
FT-IR measures all compounds in a mixture at the same time (one spectrum can yield concentrations for dozens of components)
Relationship Between Antioxidant Depletion and Oxidation
Goal: Predict oxidation before the critical saturation point is reached
Performed a modified thermal oxidation stability test (TOST D943)
• Universal Oxidation Test (D6514, D5846)• Similarity to RPVOT (D2272)
Started with 250mL used Chevron ISO 32 turbine oil
• heated in a lab oven at 135°C • Cu and Fe catalysts added
2mL removed and tested every 2-3 days
• Experiment lasted 26 days• Trended phenolic, aminic and oxidation FT-IR measurements over time
Oxidation Experiment
Relationship Between Antioxidant Depletion and Oxidation
0.00
20.00
40.00
60.00
80.00
100.00
120.00
NewISO 32
Oil
Day 1 Day 2 Day 5 Day 6 Day 8 Day 9 Day 12 Day 13 Day 16 Day 19 Day 22 Day 23 Day 24 Day 26
Phe
nolic
and
Am
inic
Ant
ioxi
dant
s (%
of
Con
c. in
New
Oil)
0.00
5.00
10.00
15.00
20.00
25.00
Oxi
dati
on (
Pea
k A
rea
Abs
orb
ance
)
Aminic Antioxidant
Phenolic Antioxidant
Oxidation
PASS MONITOR FREQUENTLY CHANGE IMMEDIATELY
Critical Saturation of Oxidation Products
Relationship Between Antioxidant Depletion and Oxidation
Phenolic Diminishes 40% right away
• Evaporation, Low molecular weight flash off
Aminic stays above 80% until near the end of useful life
Aminic Stages of depletion
• Stage 1: Initial slight increase to 110%• Stage 2: Mid-way point in oil lifespan, 25% depletion• Stage 3: Decent from 80% to 40% after phenolic reaches 30%
0.00
20.00
40.00
60.00
80.00
100.00
120.00
NewISO 32
Oil
Day 1 Day 2 Day 5 Day 6 Day 8 Day 9 Day 12 Day 13 Day 16 Day 19 Day 22 Day 23 Day 24 Day 26
Ph
en
oli
c a
nd
A
min
ic A
ntio
xid
an
ts (
% o
f C
on
c.
in N
ew
Oil
)
0.00
5.00
10.00
15.00
20.00
25.00
Ox
ida
tio
n (
Pea
k A
rea
Ab
so
rb
an
ce)
Aminic Antioxidant
Phenolic Antioxidant
Oxidation
PASS MONITOR FREQUENTLY CHANGE IMMEDIATELY
Critical Saturation of Oxidation Products
Relationship Between Antioxidant Depletion and Oxidation
AO levels give a larger window of warning than oxidation alone
• TAN tracks oxidation• Viscosity tracks oxidation
0.00
20.00
40.00
60.00
80.00
100.00
120.00
NewISO 32
Oil
Day 1 Day 2 Day 5 Day 6 Day 8 Day 9 Day 12 Day 13 Day 16 Day 19 Day 22 Day 23 Day 24 Day 26
Phe
nolic
an
d A
min
ic A
ntio
xid
ants
(%
of C
onc.
in N
ew O
il)
0.00
5.00
10.00
15.00
20.00
25.00
Oxi
dati
on (
Pea
k A
rea
Ab
sorb
ance
)
Aminic Antioxidant
Phenolic Antioxidant
Oxidation
PASS MONITOR FREQUENTLY CHANGE IMMEDIATELY
Critical Saturation of Oxidation Products
1 Day in the Lab Experiment = 3-6 months in real world oil life (assuming 6-10y oil life)
Rapid oxidation spiked on day 22• Day 20-21
– Aminic Rapid decrease– Phenolic modest decrease– Slight growth in oxidation
Day 22-23 = 1200% increase in Oxidation• Equal to 400% increase in one month real world in-service oil life
Relationship Between Antioxidant Depletion and Oxidation
Re-additize, bleed & feed, or top-off at day 19• Outcome greatly delayed by preventive
measures
Aminic perhaps the best indicator for Oxidation
Varnish layer in Day 22 and later samples• 65-80% more oxidation products in varnish
compared to free flowing oil
Relationship Between Antioxidant Depletion and Oxidation
Oxidation products are complex mixtures
Oxidation carbonyls are the most visible
• 1700-1720cm-1 region of IR spectrum
– Ketones
– Carboxylic Acids
– Aldehydes
– All contribute to the strong band at 1713cm-1
Relationship Between Antioxidant Depletion and Oxidation
C O O H
C H O
O H
O
Relationship Between Antioxidant Depletion and Oxidation
184
4.7
177
8.5
17
13.2
165
3.9
151
5.0
2020 2000 1980 1960 1940 1920 1900 1880 1860 1840 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 1620 1600 1580 1560 1540 1520 1500
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
Wavenumber
Abs
orba
nce
184
4.7
177
8.5
17
13.2
165
3.9
151
5.0
2020 2000 1980 1960 1940 1920 1900 1880 1860 1840 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 1620 1600 1580 1560 1540 1520 1500
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
Wavenumber
Abs
orba
nce
184
4.7
177
8.5
17
13.2
165
3.9
151
5.0
2020 2000 1980 1960 1940 1920 1900 1880 1860 1840 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 1620 1600 1580 1560 1540 1520 1500
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
Wavenumber
Abs
orba
nce
Oxidation,Ester
Preventive Action Zone
Oxidation at Critical Saturation Point (Day 22)
Day 23, Oxidation
AminicAntioxidant
Oxidation Carbonyl,Ketone, Carboxylic Acid, Aldehyde
Oxidation,Perester,Organic Carbonate,Cyclic Anhydride,Acid Chloride
184
4.7
177
8.5
17
13.2
165
3.9
151
5.0
2020 2000 1980 1960 1940 1920 1900 1880 1860 1840 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 1620 1600 1580 1560 1540 1520 1500
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
Wavenumber
Abs
orba
nce
184
4.7
177
8.5
17
13.2
165
3.9
151
5.0
2020 2000 1980 1960 1940 1920 1900 1880 1860 1840 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 1620 1600 1580 1560 1540 1520 1500
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
Wavenumber
Abs
orba
nce
184
4.7
177
8.5
17
13.2
165
3.9
151
5.0
2020 2000 1980 1960 1940 1920 1900 1880 1860 1840 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 1620 1600 1580 1560 1540 1520 1500
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
Wavenumber
Abs
orba
nce
Oxidation,Ester
AminicAntioxidant
Oxidation Carbonyl,Ketone, Carboxylic Acid, Aldehyde
Oxidation,Perester,Organic Carbonate,Cyclic Anhydride,Acid Chloride
Ester Oxidation products absorb at 1720-1745cm-1• Shoulders on main COOH and ketone components
1778cm-1 band likely a perester and/or an organic carbonate
Relationship Between Antioxidant Depletion and Oxidation
O
O
O
O
O
O O
O
Best practice to save new oil to compare to used in-service oil• Enables % deletion of additives• Enables top offs and/or bleed and feed
Agilent Technologies MicroLab Software stores the IR spectrum of new oil• Overlays new oil spectrum with used oil• Provides weight percent value for aminic and phenolic antioxidants,
Oxidation % of upper limit
Relationship Between Antioxidant Depletion and Oxidation
Yellow “Monitor Frequently” warning when AO’s reach critical depletion points
Red “Change Immediately” warning = critical thresholds reached
Phenolic and aminic are red = critical saturation point is imminent
Water is measured in ppm with yellow at 200ppm and red at 400ppm
Relationship Between Antioxidant Depletion and Oxidation
Remaining Useful Life Measurements- Portable FT-IR and Other Methods
Remaining Useful Life (RUL)
• Rotating Pressure Vessel Oxidation Test (RPVOT)
– Specifically designed to measure RUL
– Overall quality of oil base stock and additives
– Accelerates oxidation reaction with heat, pressure, and pure oxygen
– Requires lab set up• Pure Oxygen gas cylinders (highly flammable)• Significant sample prep and monitoring• Expensive initial costs
Remaining Useful Life Measurements- Portable FT-IR and Other Methods
RUL by Voltammetry
• Measures antioxidants
– Not a direct measurement, introduces errors • Electrochemical Technique: electrodes
– Current flow relative to applied voltage• Need new oil to compare to used oil
– Mixed oil and contaminant interference
– Wear metals, basic detergents, antifoam, and oxidation products (organic salts) cause interference
RUL by Voltammetry
Require Sample Preparation
• Careful Pipetting• Extraction step with an electrolyte solution
– Assumes a contsant extraction efficiency
– Extraction efficiencies are variable for additives in oil
– Some additives remain in oil = not measured
Voltammetric Electrodes need maintenance
• Cleaning and conditioning in buffer solutions
Remaining Useful Life Measurements- Portable FT-IR and Other Methods
RUL by 4500 Portable FT-IR
• Exact direct measurements
– Aminic antioxidant wt.%
– Phenolic antioxidant wt.%
– Oxidation Products: via carbonyl bands
– Water (ppm), calibrated to Karl Fischer
– Nitration Products: occur in electrostatic spark conditions and combustion engine oils
• Single drop of neat oil + ~30seconds to scan
– KF precision water requires a stabilizer additive
Remaining Useful Life Measurements- Portable FT-IR and Other Methods
RUL by Portable FT-IR
• No interference by wear metals or other contaminants• New oil comparison is recommended but not necessary
– Weight percent values are absolute regardless of mixed oils• Monitors the effectiveness of top-offs, bleed & feed, filtration, re-
additization, and dehydration
Remaining Useful Life Measurements- Portable FT-IR and Other Methods
Detects common contaminations of oil• Phosphate ester (EHC) hydraulic in turbine, gear, or mineral
based hydraulic fluids• “Fingerprint” IR bands for EHC• Cross contamination from filtration & dehydration units• ID and/or Quantify a range of impurities
– WD-40 in Hydraulics– High particulates like Soot, Dirt, and wear metals
Contaminant Detection By Portable FT-IR
Contaminant Detection By Portable FT-IR
0
.2
.4
.6
3500 3000 2500 2000 1500 1000
0
.2
.4
.6
3500 3000 2500 2000 1500 1000
Absorbance / Wavenumber (cm-1)
0
.2
.4
.6
3500 3000 2500 2000 1500 1000
0
.2
.4
.6
3500 3000 2500 2000 1500 1000
Absorbance / Wavenumber (cm-1)
EHC, Phosphate Ester Bands
Methods Development – Portable FT-IR
• Other 4500 FT-IR Water Calibrations
• Gear Oil• EHC hydraulic, phosphate
ester based• Polyester hydraulic, Quintolub• Grease• Crankcase oil
• ASTM E-2412 method also available for condition monitoring of mineral oil basestocks
• ASTM D2668 Transformer Oil Phenolic
Conclusions
4500 Portable Full Scanning FT-IR Spectrometers Can Provide:
• Independent phenolic and aminic antioxidant concentrations• Fast results – helps prevent an unscheduled shutdown• Eliminates calibrating, maintaining, and conditioning other systems
require• Total analysis time per sample less than 3 minutes, immediately ready
for the next sample