Desirable Properties of Paints
As pointed out before, PSP or TSP is prepared by dissolving a luminescent dye and a polymer binder in a solvent solution; the resulting mixture is then applied on a surface by spraying, brushing or dipping. After the solvent evaporates, a thin coating of the paint remains on the surface, in which the luminescent molecules are immobilized in the polymer matrix. The polymer binder is an important ingredient of the paint adhering to the surface of interest. In some cases, the polymer matrix is only a passive anchor; in other cases, the polymer may significantly affect the photophysical behavior of the paint through complicated interaction between the luminescent molecule and the macromolecule of the polymer. Since how the polymer affects the photophysical processes in the paint is not well understood, it is basically a trial and error process to find an optimal combination of a luminophore and a polymer. A good paint (PSP or TSP) for aerodynamic applications should have certain required physical and chemical properties. The following discussion is focused on the required properties of PSP while some requirements are generally applicable to TSP. A general strategy for the development of improved PSP formulations was proposed by Benne et al. (2002).
The Stern-Volmer coefficients of PSP should be chosen to match the pressure range on a tested article and the performance requirements of a photodetector (e. g. CCD camera) used in particular measurements. A large Stern-Volmer coefficient B(T) generally indicates a good pressure response. However, for aerodynamic experiments at high pressures, a large Stern-Volmer coefficient B(T) of PSP may cause unwanted severe oxygen quenching in the ambient reference conditions, considerably reducing the luminescent emission from the paint and therefore the signal-to-noise ratio (SNR) of a photodetector.
The luminescent emission of a luminophore is characterized by the quantum yield (or efficiency); it is generally desirable to have as high a luminescent output as possible to maximize the SNR of a photodetector. The luminescent intensity is proportional to the concentration of the probe molecules over a certain range. However, it cannot be increased indefinitely by increasing the dye concentration; if the concentration is too high, self-quenching of luminescence occurs. Similarly, the luminescent intensity is no longer linearly proportional to the excitation light intensity at a very high excitation level, and eventually it saturates when the excitation light intensity increases further.
Ideally, the luminescent intensity of PSP should not change with time under excitation. Usually, the luminescent intensity decreases with time due to photodegradation of a luminophore (Egami and Asai 2002). A decrease in the luminescent intensity could also be due to the presence of certain chemicals other than oxygen that can quench the luminescence. The polymer binder undergoes aging, which can change its characteristics with respect to the oxygen solubility and diffusivity. As a result, the Stern-Volmer coefficients of PSP may be altered.
The response time of PSP is mainly determined by oxygen diffusion through a paint layer when the luminescent lifetime is much shorter than the diffusion timescale. The high porosity of the paint will increase the time response. The need for fast time response depends on a particular application; a short response time of PSP is required for unsteady aerodynamic measurements. For steady-state measurements, however, the use of a fast-responding PSP does not necessarily offer an advantage. For a highly oxygen-permeable PSP with a short response time, the Stern-Volmer coefficient B(T) is usually large, and thus weak luminescence of PSP in the ambient conditions may lead to a low SNR.
A good PSP should have a weak temperature effect. The temperature sensitivity arises from two sources: the intrinsic temperature dependency of a luminophore and the temperature dependency of the solubility and diffusivity of oxygen in a polymer matrix. The latter is a major contributor to the temperature sensitivity of PSP.
The physical properties of a polymer binder, such as adhesion, hardness, coating smoothness and thickness, should be considered prior to a test. Adhesion should be strong enough to sustain skin friction on a surface particularly in high-speed flows, which is related to surface tension, solvent softening and chemical bonding. Hardness primarily depends on the type of polymers, the molecular weight and the degree of cross-linking. For example, silicone rubbers (or RTVs) are generally soft, whereas Acrylates and methacrylates are generally hard. Smoothness depends primarily on paint itself and application techniques; for most paints uniform leveling of the paint is essential to a smooth finish. The coating thickness is very dependent on application techniques for both the basecoat and PSP topcoat. It is generally desirable to minimize the coating roughness and thickness to avoid any effect on the aerodynamic characteristics of a model. Typically, the maximum rms roughness of a coating should be less than 0.25 |jm, and the coating thickness ranges from 20 to 40 |jm.
Toxicity of paint is a major concern of safety; toxic solvents such as chlorinated solvents should be avoided. Painter must be protected against contact with paint spray through the use of fresh air breathing equipment and adequate ventilation. The paint must be easily sprayed, leveled, and cured to give the specified physical characteristics of coating under different environmental conditions in wind tunnels. The solvent evaporation rate must be controlled under different conditions of temperature and humidity. Since the wind tunnel time is expensive, application of the paint should be as fast as possible. The curing temperature must be reasonable (less than 100oC); if the curing temperature is too high, it is difficult to achieve uniform curing over different metal materials. In addition, paint removal and reapplication on a model is a practical issue in wind tunnel testing.
Some paints, particularly those designed for good and robust adhesion, are difficult to remove and generally require an aggressive paint stripper like methylene chloride. This introduces problems with toxicity and insuring adequate ventilation.