A novel pyrene based binary pressure sensitive paint with low temperature coefficient and improved stability.pdf

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Sensors and Actuators B 138 (2009) 283–288
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A novel pyrene-based binary pressure sensitive paint with low temperature
coefficient and improved stability
Bharathibai J. Basu
a,∗
, N. Vasantharajan
a
, Channa Raju
b
a
b
Surface Engineering Division, National Aerospace Laboratories, Bangalore 560017, India
Experimental Aerodynamics Division, National Aerospace Laboratories, Bangalore 560017, India
a r t i c l e
i n f o
a b s t r a c t
Pyrene-based pressure sensitive paints (PSP) have certain advantages due to their high pressure sensitivity
and low temperature coefficient but their major drawback is the paint degradation under wind tunnel
conditions. This is due to the loss of pyrene as a result of diffusion and sublimation from the coating.
We have developed a novel stable binary PSP formulation in which pyrene is covalently bonded to the
polymer binder so that paint degradation is prevented. The coating thus obtained is a siloxane-based
hybrid organic–inorganic material. The pressure sensitive paint also contains an additional reference
luminophore, which is insensitive to pressure but sensitive to intensity variations on the model surface.
The second luminophore is incorporated in the paint to correct for the excitation intensity variations
during the wind tunnel experiment. The temperature coefficient of second luminophore exactly cancels
the temperature coefficient of the pressure sensitive luminophore, thereby resulting in a binary paint
with negligible temperature sensitivity.
© 2009 Elsevier B.V. All rights reserved.
Article history:
Received 17 November 2008
Received in revised form 22 January 2009
Accepted 13 February 2009
Available online 24 February 2009
Keywords:
Pressure sensitive paint (PSP)
Luminophore
Pyrene
Covalent bonding
Pressure sensitivity
Temperature coefficient
1. Introduction
The pressure sensitive paint (PSP) technique is an experimen-
tal method for the quantitative measurement of surface pressure
on wind tunnel models
[1–10].
PSP technique has some impor-
tant advantages over the conventional method of pressure taps that
are installed at discrete points on the model in a wind tunnel. The
main advantage of PSP is the high spatial resolution so that a com-
plete pressure mapping of the entire surface of the model can be
obtained, whereas pressure taps provide data only at pre-selected
discrete points. PSP is based on the principle of dynamic quenching
of luminescent molecules by oxygen. It contains luminescent sensor
molecules embedded in a transparent oxygen-permeable binder.
The paint is applied as a thin coating over the wind tunnel mod-
els. When the paint coating is illuminated with light of appropriate
wavelength, the sensor molecules become excited electronically to
a higher energy state. The molecules undergo transition back to
the ground state and they emit luminescence. Some of the excited
molecules return to the ground state by colliding with an oxygen
molecule and this process is known as oxygen quenching. The rate
of quenching is proportional to the local oxygen partial pressure,
which in turn is proportional to the air pressure. Thus the lumines-
cence of the PSP coating varies inversely with the local air pressure
on the surface of the coating.
∗
Corresponding author. Tel.: +91 80 25086251; fax: +91 80 25210113.
E-mail address:
bharathi@css.nal.res.in
(B.J. Basu).
0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.02.029
Paint is an important key element of PSP technique. Con-
ventional pressure sensitive paints are normally prepared by
simply mixing and dispersing photo-luminescent materials hav-
ing oxygen-quenching properties in oxygen-permeable polymer
binders
[1–7].
The luminescent materials used for PSP may be
broadly classified into three categories: ruthenium dyes, metallo-
porphyrins, pyrene or pyrene derivatives. These luminescent
molecules are excited in the ultraviolet or in the visible range and
they emit luminescence in the visible region. Pyrene-based PSPs
have a special advantage as they have an inherently lower temper-
ature sensitivity compared to other PSPs based on ruthenium dyes
and porphyrin dyes.
But pyrene-based paints have a major drawback of significant
paint degradation under wind tunnel conditions due to the loss of
pyrene by sublimation from the coating
[7–9,11].
Pyrene-based pressure sensitive paints are often prepared by
mixing pyrene solution with silicone polymer solution as well as
other constituents such as cross-linker, catalyst, pigment, solvent,
etc. But the fluorescence intensity of these paint coatings degrades
steadily with time and rapidly during wind tunnel experiment
[7–9].
We have studied the mechanism of degradation of pyrene-
based paint coatings and found that paint degradation is due to
diffusion and sublimation of pyrene from the coating
[11].
It is well
established that pyrene excimer formation is a diffusion-controlled
process
[12].
It is also known that the diffusion coefficient of pyrene
in silicone matrix is very high
[12–14].
It has been reported that
pyrene has mobility in polymers like polydimethyl siloxanes since
these polymers have high free volume
[13].
Chu and Thomas have
284
B.J. Basu et al. / Sensors and Actuators B 138 (2009) 283–288
calculated the diffusion coefficient of pyrene in PDMS resins with
high molecular weight and found it to be high. The comparatively
long Si–O and Si–C bonds reduce steric conflict, which facilitates
freedom of rotation of methyl group about the Si–O and Si–C bonds.
This unique structural feature leads to a significant free volume in
bulk PDMS. The diffusion of pyrene does not require the movement
of the whole macromolecule, but a rotation of the side groups of the
resin is sufficient for solute diffusion. The same is true for pyrene in
silicone coatings also. The pyrene molecules are in constant motion
within the bulk of the silicone coating and when they reach the
surface they undergo evaporation.
Therefore the diffusion and sublimation of pyrene from the paint
coating has to be prevented to enhance paint stability. Various
methods have been employed to improve paint stability. One of
the methods is to substitute pyrene by a suitable pyrene derivative
[15–17].
But most of the commercially available pyrene deriva-
tives do not exhibit excimer emission when they are embedded
in silicone matrix
[15].
Earlier we had prepared a pressure sensi-
tive paint using a pyrene derivative (1-decanoyl pyrene butanoate)
synthesized by us
[16].
This coating had exhibited high quantum
efficiency for its excimer emission and a pressure sensitivity of
about 70%/bar but the coating on model showed a 10% decrease
in fluorescence intensity after 10 wind tunnel blowdowns in wind
tunnel. Researchers at ONERA have synthesized new pyrene deriva-
tives and studied their photophysical properties
[17].
But these
compounds also were not suitable for PSP application as none of
these compounds were found to meet all the important PSP require-
ments like high quantum efficiency for excimer emission, high
pressure sensitivity and good stability.
Thus the paint degradation observed during wind tunnel test is
a major defect of pyrene-based paints prepared by simply mixing
and dispersing pyrene or pyrene derivative in a suitable oxygen-
permeable polymer binder. Therefore there is a definite need for
preventing paint degradation by immobilization of pyrene on the
binder polymer. Engler et al.
[8]
have described the limitations
of conventional PSPs at low speed conditions. The required pres-
sure accuracy is about 0.1% in the typical low speed range between
800 and 1000 mbar. This high accuracy is hard to reach because
of the errors related to PSP technique. The main source of error
is the temperature changes on the PSP-coated model. Therefore,
a temperature-insensitive paint formulation should be used to
achieve high accuracy of results in low speed flows. Their studies
have shown that their pyrene-based paint formulation was suitable
for low speed flows because of its high pressure sensitivity but the
drawback of the paint was significant degradation of the pressure
sensitive component occurring under flow conditions. Le Sant and
Merienne
[9]
have reported a pyrene-based binary paint contain-
ing pyrene and a reference luminophore of gadolinium oxysulfide
having very low temperature sensitivity but this paint also suffered
degradation in the course of wind tunnel test.
In our approach, pyrene is covalently bonded to the PDMS sil-
icone binder by using a silane-coupling agent, so that it does not
undergo diffusion and sublimation. The stability of this novel pres-
sure sensitive paint was assessed by wind tunnel aging tests. It was
found that there was no change in the fluorescence intensity (blue
emission due to pyrene excimer) of the coating after 50 wind tunnel
blowdowns in wind tunnel.
2. Experimental
2.1. Materials required
Pyrene butanol, tin octoate, dibutyl tin dilaurate (DBT) were
procured from Sigma–Aldrich. Isocyanatopropyltriethoxysilane
(ICPTES) was from Lancaster. PDMS resins were procured from
ABCR GmbH, Germany. Dichloromethane (AR), toluene (AR), ace-
Fig. 1.
Different layers of coating the model surface with PSP.
tone, xylene and butyl acetate were from Merck. Red phosphor
was procured from M/s Phosphor Technology, UK. All the chemicals
were used as received.
2.2. Instrumentation
Calibration of the PSP coupons was carried out in the cal-
ibration chamber of NAL PSP System. It consists mainly of an
excitation source, calibration chamber and detector. Excitation light
is provided by a pulsed xenon source filtered in the ultraviolet
range 335
±
5 nm, which is capable of simultaneously exciting both
luminophores in the paint coating. The calibration chamber has
provision for controlling pressure from 0.10 to 1.60 bar and temper-
ature from 10 to 60
◦
C. Two peltier-cooled scientific grade 12-bit
CCD cameras with resolution of 1280
×
1024 pixels and equipped
with suitable filters are used as detectors to acquire images of the
blue and red emission from the coating. The data analysis was done
by dedicated software of the NAL PSP System. A dry film thickness
gauge, QuaNix Keyless was used to measure the thickness of paint
coating on the model and coupons.
2.3. Preparation of PSP formulation and paint coupons
The novel PSP formulation was prepared by mixing appropriate
amounts of pyrene end-labeled polydimethyl siloxane, DBT and a
europium-doped rare earth oxysulfide as reference luminophore.
The details of the procedure for the preparation of the paint are
described elsewhere
[18].
Toluene is used as solvent to dilute the
paint mixture.
The pressure sensitive paint typically consists of three layers as
shown in
Fig. 1.
They are: (i) screen layer, (ii) adhesive layer, and
(iii) active layer. The screen layer is a white basecoat, which creates
an optical uniformity on the model surface, enhances the emission
intensity signal of the paint and is independent of the model mate-
rial. The adhesive layer is applied to ensure adhesion between the
screen layer and the active layer. The topcoat is the active layer or
pressure sensitive layer. A commercially available two-component
epoxy paint was used as screen layer as it was found to have good
solvent resistance and adhesion to the substrate surface. The thick-
ness of screen layer was in the range of 10–20 m. A silicone primer
GE SS 4044 was applied over the screen layer to improve adhesion of
the active layer. The thickness of adhesive layer was about 2–5 m.
PSP coupons and PSP coating on model were prepared by spray-
ing the PSP formulation on the model surface and the aluminum
coupons pre-coated with screen layer and adhesive layer.
The model and coupons were cleaned with acetone or a clean-
ing agent containing acetone, xylene and butyl acetate and coated
with screen layer and adhesive layer. It is very important that each
layer should be completely cured before application of the next
layer. Therefore, the screen layer and primer layer were cured in an
oven at 60
◦
C for 1 h before spraying the PSP formulation. After cur-
ing the PSP layer for 24 h under ambient conditions, the coupons
were cut into 3 cm
×
3 cm size. A small amount of talc was applied
B.J. Basu et al. / Sensors and Actuators B 138 (2009) 283–288
285
on the coating to protect it from dust during wind tunnel blow-
downs. The thickness of active layer was about 30–40 m so that
the total thickness of paint layers was in the range of 50–60 m. The
paint coupons were used for calibration of the paint, determination
of temperature coefficient and for thermal aging test. The coating
sprayed on model was used for carrying out wind tunnel aging test.
2.4. Preparation of calibration graph and determination of
pressure sensitivity and temperature coefficient of PSP coating
The coupon was placed in the PSP calibration chamber and the
excitation radiation from the source was focused on the coating.
The pressure in the calibration chamber was changed in the range
of 0.20–1.20 bar in steps of 100 mbar at a constant temperature of
25
◦
C. Emission intensity images from the coating in blue region
(450–550 nm) and red region (600–650 nm) were recorded using
two CCD cameras with appropriate filters. The normalized inten-
sity,
I,
was determined by taking the ratio of blue and red intensity
at each pressure value. The ratio of normalized intensity,
I
ref
/I, was
calculated, where
I
ref
was the normalized intensity at the refer-
ence pressure.
P
= 1 bar was taken as the reference pressure,
p
ref
.
The Stern–Volmer calibration graph was prepared by plotting the
intensity ratio
I
ref
/I versus the pressure ratio,
p/p
ref
. The pressure
sensitivity of the binary paint was obtained from the slope of the
linear fit of the calibration graph.
The temperature inside the calibration chamber was varied in
steps of 5
◦
C and the measurements were made after equilibrating
the coating for a period of 5 min at each temperature. The inten-
sity ratio of blue to red emission (I
blue
/I
red
) was determined at each
temperature. The intensity ratio was normalized with reference to
the intensity ratio at the reference temperature (20
◦
C) and plotted
as a function of temperature.
3. Results and discussion
3.1. Photophysical properties and pressure sensitivity of the PSP
coating
The fluorescence spectrum of the PSP coating exhibits two types
of emissions of pyrene; monomer peak centered around 400 nm
and excimer peak at about 480 nm (Fig.
2).
The third peak at 615 nm
is the europium emission of the red phosphor. The excimer emis-
sion of pyrene is quenched by oxygen as shown in
Fig. 2.
It is also
sensitive to changes in air pressure and is made use for PSP appli-
cation.
The relationship between fluorescence intensity of the paint
coating and pressure on the surface of the coating is given by the
Fig. 3.
Effect of air pressure on the blue and red intensity of the binary PSP coating.
following equation, which is a modified form of Stern–Volmer equa-
tion
[1–7]:
I
ref
=
A
+
B
I
P
P
ref
,
where
I
ref
and
I
are the fluorescence intensity of the paint coating
at air pressure,
P
and reference pressure,
P
ref
.
A
and
B
are constants.
P
= 1 bar is normally taken as reference pressure in PSP measure-
ments.
Fig. 3
shows the variation of blue and red intensity with air pres-
sure. It can be seen that blue intensity decreased exponentially with
pressure, whereas red intensity was pressure-independent and
nearly constant. The calibration graph of the binary paint coating
is shown in
Fig. 4.
Very good linearity with a regression coefficient
of 0.9998 was obtained with a pressure sensitivity of 69.5%/bar.
Reproducible results were obtained with different batches of the
paint formulations.
The Stern–Volmer calibration plots of the binary paint at three
different temperatures, 10, 20 and 30
◦
C are shown in
Fig. 5.
It can
be seen that the calibration graphs at different temperatures match
very well in the pressure range of 0.8–1.2 bar and deviate slightly
at lower pressures.
Fig. 2.
Fluorescence spectra of the novel binary PSP coating in the presence of air
and nitrogen.
Fig. 4.
Calibration graph of the binary PSP coating at
T
= 25
◦
C.
286
B.J. Basu et al. / Sensors and Actuators B 138 (2009) 283–288
Fig. 5.
Calibration graph of the binary PSP coating at different temperatures, 10, 20
and 30
◦
C.
Fig. 6.
Effect of temperature on the normalized intensity ratio of the binary PSP
coating at
P
= 1 bar.
3.2. Temperature sensitivity
It is well known that PSPs are sensitive not only to pressure,
but also to temperature
[1–10,19–26].
Due to this tempera-
ture sensitivity, pressure cannot be accurately determined from
the luminescence intensity of the PSP. Temperature dependence
has been cited as the largest source of error in PSP mea-
surements. Several inventors have attempted to correct for the
temperature sensitivity of PSPs
[19–26].
PSPs prepared using
platinum porphyrins normally exhibit high temperature sen-
sitivity. The errors caused by temperature sensitivity of such
paints are reduced or eliminated using dual luminophore PSP
or binary PSP in which a second temperature sensitive lumines-
cent dye is incorporated into the sensor film
[22–26].
Kose et
al.
[22]
have prepared dual luminophore coating consisting of
the oxygen sensitive platinum tetra(pentafluorophenyl)porphyrin
(PtTFPP) and temperature sensitive ruthenium phenanthroline
complex and they have used the emission from temperature sen-
sor to correct for the temperature dependence of the pressure
response of the PtTFPP sensor. They have also reported a differ-
ent calibration method based on principal component analysis
of luminescent images of the emissions of both luminophores to
measure surface air pressure with higher accuracy compared to
the intensity-ratio method
[23].
Another dual luminophore PSP
with PtTFPP utilizes Rhodamine B as the temperature sensor to
correct the temperature dependence of PtTFPP
[24].
Khalil et al.
[25,26]
have developed a dual-luminophore PSP with oxygen sen-
sitive platinum tetra(pentafluorophenyl)porpholactone (PtTFPL)
and magnesium tetra(pentafluorophenyl)-porphyrine (MgTFPP) as
pressure-independent reference in FIB polymer which is reported
to produce ideal PSP measurements with a very low temperature
dependency of
−0.1%/
◦
C.
As mentioned earlier, pyrene-based paints exhibit a lower
temperature coefficient (of about 0.3%/
◦
C) compared to single
luminophor paints based on platinum porphyrins and ruthenium
dyes. But it is preferred to use a paint with still lower temperature
coefficient to achieve greater accuracy of pressure measurement
especially for low speed applications. The most promising solu-
tion to this problem is to use a binary paint with the second
luminophore having the same temperature sensitivity as the active
luminophore. In our paint formulation, the temperature sensitivity
of active luminophore is compensated by the temperature sensitiv-
ity of the reference luminophore thereby resulting in a binary paint
with negligibly low temperature sensitivity at
P
= 1 bar.
The plot of normalized intensity ratio as a function of tempera-
ture at
P
= 1 bar is shown in
Fig. 6.
It can be seen that the temperature
coefficient is negligibly low (<0.10%/
◦
C) in the temperature range of
10–50
◦
C.
3.3. Stability of the paint coating
The paint stability was assessed by conducting a static thermal
aging test. The PSP coupons were calibrated and then subjected
to a temperature of 60
◦
C and 0.10 bar for 1 h in a vacuum oven
and calibrated again. The intensity and pressure sensitivity of the
coupons of the novel PSP (PSP-1) before and after thermal aging test
were compared. For comparison, a paint coupon prepared by mix-
ing pyrene solution in silicone resin (PSP-2) was also subjected to
thermal aging test. Several paint coupons of both types were eval-
uated for their stability by thermal aging test and the average of
five measurements were taken.
Fig. 7
shows a comparison of the
normalized intensities of both paint coupons before and after ther-
mal aging test. It was found that the change in normalized intensity
Fig. 7.
Comparison of the effect of thermal aging on the normalized intensity of the
novel PSP coating (PSP-1) and the simple pyrene-based coating (PSP-2).
B.J. Basu et al. / Sensors and Actuators B 138 (2009) 283–288
287
Fig. 8.
Comparison of the calibration graphs of the binary PSP coating before and
after thermal aging test at 60
◦
C and 0.1 bar.
Fig. 10.
Comparison of the calibration graphs of the binary PSP coating before and
after wind tunnel aging test.
of the new PSP coating after thermal aging test was less than 5%,
whereas the degradation in intensity was about 60% for the sim-
ple pyrene-mixed paint coating. The calibration graphs of the novel
paint coating before and after thermal aging test is shown in
Fig. 8.
It was found that there was no change in pressure sensitivity of the
paint coating after thermal aging test.
The paint stability was further assessed through wind tunnel
aging test. Aging test of the paint coating was conducted in the
0.3 m-trisonic blowdown wind tunnel at NAL at a Mach number
of 0.8 and a model incidence,
˛
of 10
◦
. Each blowdown was of
approximately 25 s duration. The exposure time of CCD cameras
was 6 s. The blue and red images were acquired after the first blow-
down, after every 5 blowdowns and continued up to 50 blowdowns.
The methodology utilized for wind tunnel aging test comprises of
comparing the wind-off images of both blue and red emissions at
ambient conditions with those obtained after the first blowdown.
The normalized intensity maps of the images after 5–50 blowdowns
were compared with the image obtained after the first blowdown.
The normalized intensity image of the coating after 50 blowdowns
with reference to the image obtained after the first blowdown is
shown in
Fig. 9.
It was found that there was no degradation for the
paint after 50 blowdowns. A paint coupon was fixed on the model
surface as seen in
Fig. 9
and this coupon was calibrated before and
after wind tunnel aging test. The comparison of these calibration
graphs is shown in
Fig. 10.
There was practically no change in paint
calibration after the aging test in wind tunnel. Also, there was no
visible mechanical damage to the surface of the paint coating due
to blowdowns thus showing that the coating has good adhesion
and required hardness. These tests have demonstrated that paint
coating of novel binary PSP formulation with covalently bonded
pyrene derivative has improved stability and do not suffer from
paint degradation as observed in the case of conventional mix type
pyrene-based PSPs.
4. Conclusions
A novel binary PSP formulation with improved stability was pre-
pared by covalent immobilization of pyrene on the PDMS binder
so that paint degradation is prevented. The calibration graph of
the paint showed high linearity with a pressure sensitivity of
about 69–70%/bar at
T
= 25
◦
C and a low temperature coefficient of
<0.1%/
◦
C at
P
= 1 bar. The low temperature coefficient was achieved
by matching the temperature coefficients of the active and refer-
ence luminophores of the binary paint. The stability of the new
binary paint coating was established by thermal aging tests and
wind tunnel aging tests.
Acknowledgements
The authors are grateful to Dr. A.R. Upadhya, Director, NAL and
Dr. K.S. Rajam, Head, Surface Engineering Division for their support
and encouragement of this work. We thank Dr. L. Venkatakrishnan
for valuable discussions on PSP calibration and wind tunnel aging
tests of the PSP coating on the model.
References
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[2] T. Liu, B.T. Campbell, S.P. Burns, J.P. Sullivan, Temperature and pressure sensitive
luminescent paints in aerodynamics, Appl. Mech. Rev. 50 (1997) 227–246.
[3] R.C. Crites, Measurement Techniques, Von Karman Institute for Fluid Dynamics,
Lecture Series 1995–01, 1995, pp. 1–38.
[4] J. Kavandi, J. Callis, M. Gouterman, G. Khalil, D. Wright, E. Green, Luminescent
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Fig. 9.
Normalized intensity ratio image of the PSP coating on the model after 50
blowdowns in the wind tunnel; a paint coupon embedded on the model surface for
comparison of paint calibration can be seen.

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