WO2011023446A1

WO2011023446A1 - Sensitive paints

Info

Publication number: WO2011023446A1
Application number: PCT/EP2010/059957
Authority: WO
Inventors: Ingo Klimant; Sergey Borisov
Original assignee: Technische Universität Graz; Forschungsholding Tu Graz Gmbh
Priority date: 2009-08-31
Filing date: 2010-07-12
Publication date: 2011-03-03

Abstract

A sensitive paint being sensitive with respect to a physical parameter, preferably pressure, temperature, CO₂ concentration, etc., comprises a luminophor being incorporated into polymeric beads and having at least one luminescence property, e.g. intensity or decay, which depends on said physical parameter. The polymeric beads are nanoparticles comprising a hydrophobic core, containing said luminophor and a hydrophilic shell enabling dispersion of said nanoparticles in water. The luminophor contained in said hydrophobic core is e.g. a pressure sensitive luminophor, a temperature sensitive luminophor or a Luminophor being sensitive with respect to pH or ion concentration.

Description

Sensitive Paints

BACKGROUND OF THE INVENTION

The invention relates to a sensitive paint, being sensitive with respect to a physical parameter, preferably pressure, temperature, CO₂ concentration, etc., comprising a luminophor being incorporated into polymeric beads and having at least one luminescence property, e.g. intensity or decay, which depends on said physical parameter.

DESCRIPTION OF PRIOR ART

Aerodynamic structures e.g. such as aircraft components are commonly tested in a wind tunnel to gather data for use in verification of characteristics and in design improvements. Various quantities are measured in the wind-tunnel testing, including, for example, the pressure distribution at the surface of the structure. The pressure information is used to calculate air flows and force/ press u re distributions over the structure.

A number of techniques have been employed over the years to make the pressure measurements. Mechanical or electronic pressure sensors can be affixed to the external surface of the structure. Such sensors ordinarily have a portion extending above the surface into the air stream, which itself can alter the air stream and the measured values. The sensors ordinarily are relatively large in size, which limits the spatial resolution of the data that is gathered and also limits the number of sensors that can be employed. In another approach, small orifices are provided in the surface to act as pressure taps. The taps communicate at one end with the pressure at the surface of the structure and at the other end with a pressure transducer. If the structure under test is a subscale model, it is ordinarily quite difficult to use a large number of pressure taps due to the size of each tap and its pressure transducer. The spatial resolution of the measurements and the number of sensors that can be used is therefore limited. Lastly, models utilizing pressure taps are time consuming and expensive to build.

A more recently developed alternative approach is luminescence barometry, described, for example, in US Patent 5,359,887. In this technique, the surface of an aerodynamic structure is coated with a paint constituting a formulation of a binder and an active agent that emits light when excited by radiation of a particular type, such as ultraviolet or visible blue light. For some types of active agents, the presence of oxygen, such as found in the air, quenches or reduces the light emission. The extent of quenching is proportional to the partial pressure of the oxygen, or, stated conversely, the light output of the active agent is inversely proportional to the partial pressure of the oxygen. The binder is selected to hold the paint in place on the surface of the aerodynamic structure, yet permit the oxygen in the atmosphere to permeate there through and reach the active agent to perform the quenching function.

The higher the pressure of oxygen in the atmosphere contacting the luminescent paint, the lower the light emission of the paint. The intensity of light emission of the paint is therefore a useful measure of the local oxygen partial pressure, and thence the total local air pressure, of the atmosphere in contact with the paint. The light intensity is measured optically, so that the limit of spatial resolution is typically determined by the spatial resolution of the optical system. Thus, the aerodynamic structure is effectively "instrumented" for wind-tunnel pressure testing by painting the structure with the pressure-sensitive paint, illuminating the structure with the required wavelength of radiation, and measuring the luminescence and light output intensities over the surface of the model using an optical imaging system.

According EP O 830 579 Bl a pressure sensitive paint is coated onto the surface over which a pressure distribution is to be measured. A pressure sensitive paint is a mixture of a photoluminescent-compatible, oxygen-permeable binder that is a mixture of silanol-terminated polydimethylsiloxane and methyltriacetoxysilane, and a photoluminescent active agent such as tris(4,7-diphenyl-l,10-phenan- throline) ruthenium(II) chloride pentahydrate. The binder and active agent are placed into an appropriate amount of a solvent for the binder and the active agent, such as dichloromethane, and applied to a surface.

In D. M. Oglesby et al. "Water-Based Pressure Sensitive Paint" NASA/TM-2004- 213268 preparation and performance of a water-based pressure sensitive paint (PSP) is described. A water emulsion of an oxygen permeable polymer and a platinum porphyrin type luminescent compound are dispersed in a water matrix to produce a PSP that performs well without the use of volatile, toxic solvents. The primary advantages of this PSP are reduced contamination of wind tunnels in which it is used, lower health risk to its users, and easier cleanup and disposal. This also represents a cost reduction by eliminating the need for elaborate ventilation and user protection during application. The water-based PSP described has all the characteristics associated with water-based paints (low toxicity, very low volatile organic chemicals, and easy water cleanup) but also has high performance as a global pressure sensor for PSP measurements in wind tunnels. The use of a water-based PSP virtually eliminates the toxic fumes associated with the application of PSPs to a model in wind tunnels. Disadvantageous is the fact, that this water-based paint typically need at least 24 hr at room temperature to sufficiently cure for testing. After curing the paint is water insoluble, thus cleaning is only possible by sand blasting.

In F. Kimura et al., "Development and characterization of fast responding pressure sensitive microspheres" Review of Scientific Instruments 79, 074102 2008 the pressure sensitive paint (PSP) tested uses platinum tetra(penta- fluorophenyl)porphine (PtTFPP), platinum octaethylporphine (PtOEP), and a novel set of osmium-based organometallic complexes as pressure sensitive luminophors incorporated into polymer matrices of dimethylsiloxane bisphenol A- polycarbonate block copolymer or polystyrene. Two types of pressure sensitive microspheres were used, the first being PtOEP-doped polystyrene microspheres (PSBeads) and the second being porous silicon dioxide microspheres containing a pressure sensitive osmium complexes. For the microspheres, 2.5 μm diameter PSBeads showed a response time of 3.15 ms, while the osmium-based silicon dioxide microspheres showed a response time ranging between 13.6 and 18.9 _μs.

In Gouterman M, Callis J, Dalton L, Khalil G, Mebarki Y, Cooper KR, Grenier M, "Dual luminophor pressure-sensitive paint: III. Application to automotive model testing", MEASUREMENT SCIENCE & TECHNOLOGY, 2004, V. 15, pp. 1986-1994 a successful application of the use of platinum porphyrin (PtP) in pressure- sensitive paint (PSP) is disclosed. Oxygen in the film quenches luminescence, and oxygen pressure can be monitored by measuring the ratio of I(wind- off)/I(wind-on). But this ratio is compromised if there is model motion and if the paint layer is inhomogeneous. Furthermore it requires careful monitoring and placement of light sources. Moreover, this method is seriously affected by temperature. The errors caused by model motion and temperature sensitivity are eliminated or greatly reduced using dual luminophor paint. The PSP is made from an oxygen sensitive luminophor, Pt tetra(pentafluorophenyl)-porpholactone and Mg tetra(pentafluorophenyl)porphine, which provides temperature-sensitive paint (TSP) as the pressure-independent reference.

In Puklin E, Carlson B, Gouin S, Costin C, Green E, Ponomarev S, Tanji H, Gouterman M, "Ideality of Pressure-Sensitive Paint. I. Platinum Tetra(penta- fluorophenyl)porphine in Fluoroacrylic Polymer", JOURNAL OF APPLIED POLYMER SCIENCE, 2000, V. 77, pp. 2795-2804 the pressure sensitive paint (PSP) properties of a fluoroacrylic polymer, FIB, with the luminophor platinum tetra(pentafluorophenyl)porphine (PtTFPP) are presented. This paint forms a hard coating that displays Stern-Volmer plots with a high dynamic range, good photostability, a response time of less than 1 s and a relatively low temperature dependence. The temperature dependence is low because FIB has a unusually low activation energy for the diffusion of oxygen. Pressure and temperature affect intensity independently.

In S. M. Borisov, T. Mayr and I. Klimant, "Poly(styrene-block-vinylpyrrolidone) beads as a versatile material for simple fabrication of optical nanosensors", ANALYTICAL CHEMISTRY, 2008, V. 80, pp. 573-582 a versatile platform for designing optical nanosensors is proposed. The "sensing chemistries" of the optical sensors are entrapped into poly(styrene-block-vinylpyrrolidone) nano- beads having the average size of 245 nm in aqueous media. Addressable staining into the core or the shell of the beads results in nanosensors for essential analytes such as dissolved oxygen, temperature, pH, chloride, and copper ions. Two incorporating procedures are developed : staining in the polystyrene core is performed from a tetrahydrofu ran/water mixture (50: 50 v/v) and staining in the poly(vinylpyrrolidone) shell is achieved by using the ethanol/water mixture (70: 30 v/v). The oxygen and temperature indicators should be preferably incorporated into the core, whereas nanosensors for ions are manufactured by staining into the shell. In the case of the lipophilic pH indicators both procedures result in similar pKa values. There is no disclosure with respect to pressure sensitive paints.

SUMMARY OF THE INVENTION

Although the existing paints are operable, there is an ongoing need for a paint formulation that is sensitive to changes of e.g. pressure, temperature or CO₂, that is durable, and that is easily applied to produce a smooth surface on the aerodynamic structure.

It is further an object of the present invention to develop a sensitive paint, preferably a pressure sensitive paint, which can be applied and removed from an aerodynamic structure without the use of toxic and/or expensive solvents.

The invention achieves this object by polymeric beads which are nanoparticles comprising a hydrophobic core, containing said luminophor and a hydrophilic shell enabling dispersion of said nanoparticles in water.

The nanoparticles so-called core-shell-particles, having an average size of between 100 and 1000 nm, preferably 150 and 400 nm, are dispersible in water in concentrations up to 50%w/w. They may be obtained as a poly(styrene-block- vinylpyrrolidone) emulsion in water (38% w/w emulsion in water) from Aldrich (www.sigmaaldrich.com). The invention presents new and clean water based sensitive layers. Surprisingly dispersions of polymeric nanoparticles in water form rugged solid films e.g. on aerodynamic structures after evaporation of the water - without any additional binder. These films have outstanding sensing properties:

- tunable sensitivity

- fast response

- high mechanical stability

- high level of porosity

- good adhesion on many metallic or polymeric supports

- easily removable with water only

The sensitive luminophor (e.g. an oxygen sensitive indicator) is located in the hydrophobic core (e.g. polystyrene) of the polymeric core-shell-nanoparticles. The hydrophilic shell (e.g. polvinylpyrrolidone) of the particles enables the dispersion in water in concentrations up to 50% content of solids. The hydrophilic (e.g polvinylpyrrolidone) shells have very high adhesion to almost any other material including other particles. Thus it acts as an perfect internal binder and is responsible for the excellent mechanical stability of the films.

The inventive, sensitive paint can be prepared from clean water dispersions and can be also washed away with water from an aerodynamic structure.

The inventive paints are cheap, and it is possible to mix other particles to the dispersion (e.g. Cr(III)-doped solid state materials as ruby or luminescent Ru(II)-polypyridylcomplexes encapsulated into an oxygen impermeable material) for simultaneous recording of temperature).

According to a first preferred variant said luminophor contained in said hydrophobic core is a pressure sensitive luminophor, preferably an oxygen sensitive luminophor, e.g. platinum(II) 5, 10,15, 20-tetrakis-(2, 3,4,5, 6-penta- fluorophenyl)-porphyrin (PtTFPP), which can be obtained from Frontier Scientific (www.frontiersci.com). The chemical structure is disclosed in Fig. 7.

According to a second preferred variant said luminophor contained in said hydrophobic core is a temperature sensitive luminophor, preferably Eu(tta)sL (see chemical structure in Fig. 7). According to a third preferred variant said luminophor contained in said hydrophobic core is a luminophor being sensitive to an acidic or basic reacting gas, preferably carbon dioxide or a volatile amine. The used luminophor is for example CHFOE or HPTS(OA)₃ (see chemical structures in Fig. 7)

Table 1. Composition of the sensitive paints

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described below with reference to the enclosed drawings and diagrams showing in

Fig. 1 a schematic illustration of the testing of an aerodynamic structure using the inventive pressure-sensitive paint;

Fig. 2 a surface electron microscopy (SEM) image of the Poly(styrene- block-vinylpyrrolidone) beads (PS-PVP beads);

Fig. 3 images of an aluminium foil before spraying (left), after spraying

(middle) and after washing of the sprayed foil with water (right);

Fig. 4 an image of the aluminium foil covered with PtTFPP/PS-PVP beads;

Fig. 5 a schematic representation of the sensitive paint composed of the oxygen-sensitive and temperature-sensitive beads;

Fig. 6 emission spectra of the aluminium foil sprayed with a suspension containing oxygen- and temperature-sensitive beads; and

Fig. 7 chemical structures of luminophors used in the sensitive paints. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 shows in a schematic representation an aerodynamic structure 10, in this case e.g. a car model, which is undergoing testing in an air flow 11 in a wind tunnel 12. At least partially the aerodynamic structure 10 is coated with a layer

13 of inventive pressure-sensitive paint. The layer 13 is illuminated by a beam

14 of radiation produced by an excitation source 15. The illumination source 14 is preferably a source of UV light, using a suitable excitation filter 16. The beam 14 photo excites an active agent in that portion of the paint layer 13 lying within an illuminated region 17, with the light output of the active agent responsive to the local surface pressure on the surface of the car model .In a variant of the invention the layer 13 additionally comprises a temperature sensitive luminophor making the light output responsive to the local surface temperature of the cor model.

The illuminated region 17 is viewed by an imaging system 18, which includes a video camera system 19 and/or a digital camera system with an entrance filter 21, preferable a RG 630 long-pass filter. The camera system 19 includes an internal optical system that focuses the image of the illuminated region 17 onto an imaging photo detector (e.g. a CCD) The camera system 19 images the spatial distribution within the illuminated region 17 of the light intensity emitted from the pressure-sensitive paint layer 13. The image is viewed and recorded on a monitor system 20. Scanning means can be provided for both the excitation source 15 and the imaging system 18 so that regions over the entire surface of the car model 10 can be viewed and analyzed.

Fig. 4 shows an image of the aluminium foil covered with PtTFPP/PS-PVP beads excited by UV light and viewed through an RG 630 long-pass filter. The bright spot indicates an area of low oxygen tension (nitrogen is purged onto the air- equilibrated foil).

Fig. 6 shows emission spectra of the aluminium foil sprayed with a suspension contained 15% w/w of the oxygen-sensitive beads (PtTFPP in PS-PVP) and 10% w/w of the temperature-sensitive beads (Eu(tta)₃DEADIT in PViCI-PAN). λ_exc = 405 nm

Example 1

Preparation of oxygen-sensitive nanobeads: 1052 mg of the polymer emulsion (containing 400 mg of the core-shell-beads) was diluted with the mixture of 110 ml_ of water and 70 ml_ of Tetrahydrofuran (THF). Six milligrams of PtTFPP were dissolved in 30 ml_ of THF, and the solution was added dropwise under vigorous stirring into emulsion of the polymer. THF was removed under reduced pressure and the dispersion was concentrated to 2.5 ml_ overall volume.

Example 2

Preparation of temperature-sensitive nanobeads: 400mg of PViCI-PAN and 6 mg of Eu(tta)sDEADIT were dissolved in 200 ml_ of acetone. 600 mg of water was added dropwise under vigorous stirring. Acetone was removed under reduced pressure and the resulting beads were freeze-dried.

Example 3

Preparation of temperature-sensitive nanobeads: Preparation was performed according Example 1, however 6 mg of Eu(tta)sL was used instead of same amount of PtTFPP.

Example 4

Preparation of the sensitive paint: 100 mg of the temperature-sensitive beads were added to 1 g of the 15% w/w dispersion of the oxygen-sensitive beads. The mixture was spayed onto an aluminium plate to result in a mechanically stable homogeneous layer.

Claims

C L A I M S

1. A sensitive paint being sensitive with respect to a physical parameter, preferably pressure, temperature, CO₂ concentration, etc., comprising a luminophor being incorporated into polymeric beads and having at least one luminescence property, e.g. intensity or decay, which depends on said physical parameter, wherein said polymeric beads are nanoparticles comprising a hydrophobic core, containing said luminophor and a hydrophilic shell enabling dispersion of said nanoparticles in water.

2. A sensitive paint according to claim 1, wherein said nanoparticles containing said pressure sensitive luminophor are dispersible in water in concentrations up to 50%w/w.

3. A sensitive paint according to claim 1 or 2, wherein said nanoparticles containing said pressure sensitive luminophor form a rugged nanoporous film e.g. on an aerodynamic structure.

4. A sensitive paint according to any of claims 1 to 3, wherein said rugged nanoporous film is removable with water from said aerodynamic structure

5. A sensitive paint according to any of claims 1 to 4, wherein said nanoparticles are poly(styrene-block-vinylpyrrolidone) nanobeads emulsified in water.

6. A sensitive paint according to any of claims 1 to 5, wherein the average size of the nanoparticles is between 100 and 1000 nm, preferably between 150 and 400 nm.

7. A sensitive paint according to any of claims 1 to 6, wherein said luminophor contained in said hydrophobic core is a pressure sensitive luminophor, preferably an oxygen sensitive luminophor, e.g. platinum(II) 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (PtTFPP)

8. A sensitive paint according to claim 7, wherein said sensitive paint comprising Cr(III)-doped solid state materials as ruby or luminescent Ru(II)-polypyridylcomplexes encapsulated into an oxygen impermeable material) as an temperature sensitive component.

9. A sensitive paint according to any of claims 1 to 6, wherein said luminophor contained in said hydrophobic core is a temperature sensitive luminophor, preferably Eu(tta)₃L.

10. A sensitive paint according to any of claims 1 to 6, wherein said luminophor contained in said hydrophobic core is a luminophor being sensitive to an acidic or basic reacting gas, preferably carbon dioxide or a volatile amine.

11. A sensitive paint according to claim 10, wherein said luminophor is CHFOE or HPTS(OA)₃.

12. A sensitive paint according to any of claims 1 to 6, wherein said polymeric beads are a mixture of first and second nanoparticles, wherein said first nanoparticles containing a pressure sensitive luminophor and said second nanoparticles containing a temperature sensitive luminophor.