Study of the mechanism of degradation of pyrene-based pressure sensitive paints
Introduction
In recent times, pressure sensitive paint (PSP) technique has been increasingly used for the measurement of surface pressure distribution in wind tunnel studies [1], [2], [3], [4], [5], [6], [7], [8], [9]. This technique has some important advantages in comparison to the conventional methods like pressure taps. The high spatial resolution is the main advantage of the PSP technique that has attracted the attention of aerodynamic researchers all over the world. The technique is based on the principle of luminescence quenching of some materials by oxygen. The principle of PSP is the same as that of optical oxygen sensors. PSP is composed of luminescent molecules embedded in a polymeric binder and this paint can be applied over the wind tunnel models. The details of the PSP technique and the paint are described elsewhere [1], [2], [3], [4], [5], [7], [8]. The important characteristics required for a good paint are high pressure sensitivity, low temperature coefficient, high fluorescence quantum efficiency, good photo stability and fast response time. Since the paint has to function in wind tunnel conditions, it also should have good adhesion and stability.
The luminescence intensity of the paint is inversely proportional to the air pressure and directly proportional to excitation light intensity. The accuracy of the surface pressure measurements by PSP is reduced by the variation of the excitation intensity over the model surface during the test [1], [2], [3], [4], [5], [6], [7]. The error introduced by the non-uniformities of the excitation intensity can be eliminated by incorporating a second reference luminophore which is insensitive to pressure but sensitive to variations in excitation intensity. Such binary pressure sensitive paints are reported to give more accurate surface pressure data [4], [5], [6], [7].
Three kinds of luminescent molecules or luminophores are generally used for the preparation of PSPs. They are platinum porphyrin complexes [1], [2], [3], [4], [5], [7], [8], [9], [10], ruthenium di-imine complexes [1], [2], [3], [4], [5], [7], [11], [12], [13] and pyrene [6], [7], [14], [15], [16], [17]. Paints based on platinum and ruthenium dyes as the luminophores possess high temperature coefficients. Temperature sensitivity is a serious drawback of PSP that decreases the accuracy of the surface pressure measurements [8], [11], [12], [13]. Hence corrections have to be applied for the pressure data obtained by these paints. There are certain advantages in using pyrene as the active luminophore for PSP since it has relatively high quantum efficiency, good pressure sensitivity and low temperature coefficient at ambient temperatures [6], [7]. We have developed a pyrene-based binary pressure sensitive paint termed as “NAL-G” [15]. This paint contains two luminophores, pyrene as the pressure sensor and europium doped yttrium oxysulfide (Y2O2S:Eu) as the reference luminophore in a single component silicone resin. The reference luminophore has a strong phosphorescent emission in the red region. As the luminescence of pyrene is in the blue region, there is no interference between the two emissions. This paint was tested on a delta-wing-body model in 1.2 m wind tunnel at NAL. It was found that the paint had a pressure sensitivity of 75%/bar and a low temperature coefficient (<0.3%/°C). The paint cured in 24 h at room temperature and had good adhesion. The response time of the paint was fast so that it could be used in transonic flows. But NAL-G paint showed a certain degree of degradation in the blue emission intensity after a few blowdowns. Two more paint formulations with improved stability (NAL-G1 and NAL-G2) were developed subsequently [16], [17] but they also degraded after several blowdowns.
It has been reported that pyrene-based paints undergo degradation because of the evaporation of pyrene under wind tunnel blow down conditions [6], [7]. Mebarki has studied the degradation of pyrene-based PSP in the temperature range from 40 to 100 °C and found that the emission intensity of pyrene decreased at 50 °C and above [7]. Since pyrene is an organic luminophore, the degradation of the paint may occur either due to UV exposure or due to the evaporation of pyrene. We have carried out systematic investigations to understand the degradation of pyrene-based paints in wind tunnel. Thermal aging studies of the paint specimens with different pyrene concentrations were carried out to study the effect of temperature and pressure on the paint stability. This study has brought to light some interesting results and these are described here.
Section snippets
Materials
Pyrene was obtained from Acros Organics. The reference luminophore, Y2O2S:Eu (red phosphor, type QKL63/N-C1) was procured from M/s Phosphor Technology, UK. The silicone resin, RTV IS 9188 was obtained from GE Silicones and used as the binder for the paint. RTV IS 9188 is a translucent one component neutral cure silicone resin and selection of this resin as the binder for our PSP was done based on our previous investigations [15]. Toluene (sulfur-free) was used as the diluent for the paint. A
Effect of UV irradiation on the fluorescent intensity of the coating
The effect of UV exposure on the emission intensity of the paint coatings was studied. The pyrene concentration in the PSP coatings was 40 mM and these paint coupons were prepared as described in Section 2.1. Fig. 1 shows the fluorescent emission spectra of a pyrene-based binary PSP coating in air and in the presence of nitrogen. The broad emission peak at 480 nm is due to pyrene excimer and it can be seen from the figure that this peak is sensitive to oxygen. The blue emission intensity of the
Conclusions
Our study has shown that evaporation of pyrene assisted by diffusion in the coating as the major factor which contributes to the poor wind tunnel stability of the pyrene-based pressure sensitive paints. The decrease in intensity due to UV degradation was comparatively smaller. The diffusion of pyrene was found to be dependent on pyrene concentration, temperature, pressure and thickness of the coating. It was seen that loss of pyrene was accelerated at higher temperatures and lower pressures. A
Acknowledgements
The authors would like to thank Dr. B.R. Pai, Director, NAL for his support and permission to publish the work and Dr. T.S. Prahlad, Ex-Director, NAL for his constant encouragement during the course of the study. Our sincere thanks are also due to Dr. S.R. Rajagopalan, Advisor, NALTECH, Dr. (Mrs.) Indira Rajagopal, Emeritus Scientist, S.E.D. and Dr. P.R. Viswanath, Head, Experimental Aerodynamics Division, NAL for their valuable comments. The authors thank A.R. Dinesh and A. Thirumurugan for
Bharathibai J. Basu received her Masters degree in chemistry from Calicut University, Kerala, India in 1973 and PhD from Indian Institute of Science, Bangalore, India in 1995. She started her research career in National Aerospace Laboratories, Bangalore, India in 1974. She has worked in the area of chemical characterization of materials based on spectrophotometric and electrochemical methods of analysis. Research areas of her interest are trace element analysis, electroanalytical chemistry and
References (20)
- et al.
Pressure sensitive paints in aerodynamic testing
Exp. Therm. Fluid Sci.
(1995) - et al.
Surface pressure measurements using luminescent coatings
Annu. Rev. Fluid Mech.
(2001) - R.C. Crites, Measurement Techniques, Lecture Series, Von Karman Institute for Fluid Dynamics,...
- et al.
Temperature and pressure sensitive luminescent paints in aerodynamics
Appl. Mech. Rev.
(1997) - S.D. Torgerson, T. Liu, J.P. Sullivan, Use of pressure sensitive paints in low speed flows, AIAA Paper 96-2184, 1996,...
- M.C. Merienne, F. Bouvier, Vortical flow field investigation using a two component pressure sensitive paint at low...
- Y. Mebarki, Pressure sensitive paints: application in wind tunnels, ONERA NT 1998-6, 1998, pp....
Oxygen quenching of luminescence of pressure sensitive paint for wind tunnel research
J. Chem. Educ.
(1997)- et al.
Luminescent barometry in wind tunnels
Rev. Sci. Instrum.
(1990) - et al.
Surface pressure field mapping using luminescent coatings
Exp. Fluids
(1993)
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Bharathibai J. Basu received her Masters degree in chemistry from Calicut University, Kerala, India in 1973 and PhD from Indian Institute of Science, Bangalore, India in 1995. She started her research career in National Aerospace Laboratories, Bangalore, India in 1974. She has worked in the area of chemical characterization of materials based on spectrophotometric and electrochemical methods of analysis. Research areas of her interest are trace element analysis, electroanalytical chemistry and spectroscopy. Her current interest is in the development of pressure sensitive paints for wind tunnel studies.
C. Anandan obtained his MTech in materials science from Indian Institute of Technology, Kanpur in 1981 and PhD from University of Wales, College of Cardiff in 1990 for his work on Metal — amorphous silicon interface studies. He has worked at National Physical Laboratory, New Delhi, India from 1983 to 1999 in the area of thin film materials and devices and surface analysis techniques. Since September, 1999 he is with the Surface Engineering Division of National Aerospace Laboratories, Bangalore.
K.S. Rajam is currently heading the Surface Engineering Division, National Aerospace Laboratories, Bangalore, India. She holds a doctorate from Bangalore University. Her technical interests include developing surface modification technologies for aerospace and other engineering applications. She is currently engaged in electroplating, electroless plating, multilayer coatings, electroforming and pressure sensitive paints.