Fiber Bragg Distributed Chemical Sensor Arjen Boersma, Milan Saalmink, Timme Lucassen, Sjoukje Wiegersma Materials Technology TNO Eindhoven, The Netherlands
[email protected] Abstract— A distributed chemical sensor is developed by coating multiple Bragg gratings in a single glass fiber with chemical responsive coatings. The composition of the coating is tuned to the target chemicals to be measured and the optical response of the coated grating is optimized by changing the coating thickness. Specific recoaters were used to apply the responsive coatings reproducibly onto the glass fibers. The coating compositions can be based on UV curing or solvent based systems. In this paper we show the development and application of a distributed gas sensor for CO2, H2S and H2O that can be used for down-hole monitoring in the oil and gas industry. We have developed a toolbox for the design and modeling of the responsive polymer coatings, and the performance of the actual sensors corresponded well with the predicted behavior.
I.
INTRODUCTION
Optical sensors based on Fiber Bragg Gratings are already known for a long time for measuring physical parameters like temperature and strain [1]. The principle on which a FBG chemical sensor is based is an axial strain of the fiber, as a result of an environmental chemical effect that is to be detected, for example by using a polymeric coating on the fiber that deforms in the presence of a chemical compound (analyte). When the FBG stretches or shrinks under such strain, the spectral pattern of reflected light changes (fig. 1) and a shift in reflected wavelength (∆λ) is observed.
Rob Jansen, Rik Jansen, Lun Cheng Process and Instrumentation Development and Optics TNO Delft, The Netherlands
[email protected]
The changes in reflected wavelength provide quantitative information on the environmental effect [2]. The challenges in the design of such distributed chemical sensors are the design and synthesis of chemical selective responsive polymeric coatings, the processing of these coatings onto the optical fibers and the development of an optimal configuration of fiber packaging and interrogator system with sufficient sensitivity. In order to selectively measure the presence of an analyte, the polymer coating on the FBG must be tuned to this analyte and standard acrylic or polyimide coatings cannot be used. Therefore, a toolbox was developed to design the composition of the sensing polymer with respect to the target analyte. Based on this toolbox, that includes chemical and physical interactions, temperature stability and stiffness, we have synthesized polymers (polyacrylic and polyimide) from selected building blocks. Furthermore, by cross-linking these polymers the absorption efficiency and morphology of the coating can be tuned. This coating design process will be shown in this paper by the development of responsive coatings for the detection of humidity, CO2, and H2S at elevated temperatures and pressures. A major advantages of FBG sensors is the multiplexing capability [2]. This enables us to gather information about the environment at different locations, generating a ‘semi’ distributed chemical sensor. In order to achieve this distributed feature, the various responsive polymers must be applied to the different gratings located in one fiber. This requires precise recoating processes that cannot be done by simple dip-coating, but only by the use of a fiber recoater. In the application of the responsive coatings, two recoaters have been used: one for UV curable acrylic coatings and one for solvent based polyimide coatings. The coating formulations were designed to suit the processing requirements with respect to viscosity, photo-curability and curing temperature.
Figure 1. Working principle of a Fiber Bragg Grating based chemical sensor
Finally, we will show that the FBG chemical sensors can be used in several environments, with a broad range of temperatures and pressures.
II.
TOOLBOX
In the development of a FBG sensor for the detection of gasses, vapors and liquids, the coating must meet a number of requirements: •
The coating must selectively swell when exposed to the target analyte, and must show a limited crosssensitivity towards other analytes.
•
The coating must retain a high stiffness at high analyte concentrations and temperatures, to be able to transmit the swelling force onto the glass fiber.
•
The coating must have a high adhesion to the glass fiber.
•
The coating must show a suitable durability at service conditions: i.e. temperature, pressure and hydrolysis stability.
The last requirement will be met by the choice of the polymer backbone: for mild conditions acrylic coatings are most suitable, because of their ease of processing and chemical versatility; for extreme conditions polyimide coatings are preferred, because of their high temperature resistance. The third requirement will be met by choosing a suitable primer, that reacts covalently to the glass fiber, and is subsequently incorporated in the polymerizing coating. Silane based coupling agents having amino or acrylic functionalities are used for the preparation of the sensors in this paper. The first two requirements are the input for the design of the coating compositions. In order to facilitate this design, a toolbox has been developed, in which polymer compositions could be modeled with respect to chemical and physical interactions based on the Hansen solubility parameter, dipole moment and polarization differences between polymer and analyte [3,4]. The physical characteristics (glass transition temperature (Tg), Young’s modulus (E), and density(ρ) ) of the polymers are calculated from the group contributions and crosslink density [5-7]. Table I shows the results of the modeling for one acrylic polymer composition and water as the analyte. Modeling of the interaction between water and the polymer in table I results in a water solubility of 19%, a Tg of the swollen polymer of 52 °C and a Young’s modulus of the swollen polymer of 0.32 GPa. These values correspond very well with the experimental data. It shows however, that although there is a high swelling, the stiffness and glass transition temperature decrease too much for this polymer to be used for application in a FBG water sensor. Therefore, alternative compositions were designed having a higher degree of crosslinking. This reduced the swelling, but enhanced the stiffness and temperature stability.
TABLE I.
CALCULATED RESULTS FOR THE INTERACTION PARAMETERS AND PHYSICAL PROPERTIES OF THE SENSOR COATINGS
Component
Hansen Solubility Parameters
Physical Parameters E Tg f δd δp δh °C [MPa1/2] [MPa1/2] [MPa1/2] GPa ACMO 15.1 11.0 9.7 1.9 145 0 TEGDMA 16.0 10.6 7.5 4.8 20 1 THEITA 14.0 13.3 2.1 4.0 90 2 Polymer 15.3 11.1 9.1 2.9 162 0.6 H 2O 15.5 20.4 16.5 ACMO: acryloyl morpholine TEGDMA: triethylene glycol dimethacrylate THEITA: tris(2-hydroxy ethyl) isocyanurate triacrylate Polymer: ACMO:TEGDMA:THEITA = 54:36:10 f: number of crosslinkable groups in monomer
III.
RECOATING
The polymer material that is designed using the toolbox in section II must be applied to the glass fiber as a homogeneous layer. Two experimental set-ups have been used for this purpose. A. A UV curing acrylic recoater, in which the bare glass fiber is inserted into a mold, which is filled with the liquid monomer composition and cured. The thickness of the coating is determined by the mold. Centring of the glass fiber in the sensor is difficult, especially when the coatings are thick (fig. 2, left). B. A temperature curing imide recoater, in which the dissolved prepolymer is applied to the bare glass fiber in thin layers and cured. The thickness of the coating is determined by the polymer concentration in the solution and the number of layers applied to the fiber. The glass fiber is always located in the centre of the sensor (fig. 2, right).
Figure 2. Glass fiber recoaters for polyacrylic coatings (left) and polyimide coatings (right).
Fig. 3 shows two examples of recoated Fiber Bragg Gratings. Acrylic coatings are in general thicker than imide coatings, since a single layer of polyimide is only 3-5 µm, and an acrylic coating is typically 50 µm.
polymers and correlated to the toolbox. The shift in reflected wavelength (∆λ) as obtained from the fiber sensors exposed to 100% RH was correlated to the stiffness and swelling as calculated by the toolbox. A high stiffness (E) or swelling (ϕ) results in a high wavelength shift. Thus, values for E×ϕ are compared to ∆λ.The results are shown in figs. 5-7.
Water absorption @ 23 °C
30
Measured
Figure 3. Recoated fibers with an polyacrylic coating (left) and a polyimide coating (right)
The recoating procedure resulted in sensor arrays of glass fibers having multiple Bragg gratings. These gratings were made in pairs. The distance between the two gratings in a pair was approx. 20 mm. One of the gratings was recoated with the responsive polymer, the other was left bare. This grating will be used as a temperature sensor to correct the chemical sensor for temperature fluctuations during measurement.
Toolbox
20 15 10 5 0 0%
5%
EXPOSURE TESTS
The recoated fiber sensor arrays were exposed to a series of gasses, temperatures and pressures. For this purpose, we have built a high temperature, high pressure exposure chamber, in which CO2, N2 and H2O could be injected in several compositions at temperatures from 25 to 120 °C and pressures from 1 to 200 bars (fig. 4).
10%
20%
35%
50%
Figure 5. Equilibrium water absorption of responsive polymer at 23 °C having a degree of crosslinking from 0% to 50% as derived from the toolbox and from experiments.
FBG coated @ 23 °C
0,8
∆λ (nm) or Exϕ ϕ
IV.
Absorption (%)
25
Measured
0,6
Toolbox
0,4 0,2 0 0%
5%
10%
20%
35%
50%
Figure 6. Comparison of the shift in reflected wavelength (∆λ) as measured and the value of E×ϕ as predicted by the toolbox.
Figs. 5 and 6 show that although the water absorption decreases with increasing crosslink density, the signal ∆λ, increases. A lower water absorption is clearly compensated by an increasing stiffness. When changing the relative humidity, an almost linear correlation is found (fig. 7).
0,7 10%
Figure 4. High temperature, high pressure test setup for the exposure of the FBG sensor arrays to gasses and vapors
A. Water vapor exposure tests A string of gratings coated with six acrylic polymers was exposed to a changing relative humidity and temperature environment at atmospheric pressure. The change in reflected wavelength was monitored. The coating compositions all contained the same monomers, but the concentration of a trifunctional crosslinker was increased from 0 to 50%. The water absorption was measured at 23 °C and 100 °C for these
∆λ (nm)
0,6 0,5
20%
0,4 0,3
35%
23 °C
50%
0,2 0,1 0 40
60
RH (%)
80
100
Figure 7. Change in reflected wavelength upon an increase in relative humidity for a changing crosslinker concentration.
350 300
Measured Toolbox
0.2
250 150
0.1
100
Exϕ ϕ
∆λ (pm)
200
50 0
0
150 N2
H2S
CH4
100 75 50 25 PI-III
PI-II
0
Figure 9. ∆λ values for four different sensor fibers when exposed to CO2, N2, H2S and CH4 at 1 bar and 23 °C.
It is clear that the polymers selected have a high selectivity for both CO2 and H2S against N2 and CH4. Although the polymers were selected to be selective for CO2, the high sensitivity for H2S is due to the high activity of the gas at 1bar ( ~ρgas/ρmax). The maximum density of CO2 at room temperature is about seven times higher than that of H2S. Furthermore, there is a small difference between the four sensor polymers with respect to the sensitivity. PI-I seems to be the best polymer for the detection of CO2 or H2S in a nitrogen or methane environment. V.
CONCLUSIONS
We have shown that both acrylic and imide polymers can be designed, synthesized and applied to Fiber Bragg Gratings to manufacture chemical sensors. The sensitivity of the acrylic sensor for water is very high. The sensitivity and selectivity of the polyimide sensors for CO2 and H2S is lower, because this sensor was designed for more extreme conditions. Using this sensor, it is hard to differentiate between the two gasses, but a high selectivity against methane and nitrogen was obtained.
Figure 8. Measured wavelength shift (∆λ) and calculated E×ϕ for the PI-I coated FBG exposed to high temperature and pressure CO2.
From fig. 8 it is clear that upon increasing CO2 pressure, the signal also increases. However, at pressures above 100 bar the prediction and calculation are deviating. This is caused by the plasticizing effect of the CO2 in the polyimides, which is predicted in a more severe way by the toolbox than actually occurring. Furthermore, an increase in temperature results in a reduction in signal (both measured and predicted), which is caused by a lower solubility of gasses in polymers at higher temperatures.
CO2
125
PI-I
A string of gratings coated with five different polyimides was exposed to carbon dioxide and nitrogen at elevated pressures and temperatures, and exposed to hydrogen sulfide and methane at ambient temperatures and pressures. Four coating compositions contained 4,4’-(hexafluoro isopropylidine) diphtalic anhydride as one monomer, and a different second monomer (respectively 2,2’-bis-[4-(4-amino phenoxy) phenyl] hexafluoropropane (PI-I), 2,5-bis-(4-amino phenyl)-1,3,4-oxadiazole (PI-II), 2,2’-bis(aminophenyl) hexa fluoropropane (PI-III) and amino terminated polydimethyl siloxane (PI-IV). The fifth polymer was composed of 4,4’oxydianiline and pyromelletic anhydride (PI-V). The fluor and siloxane moieties were incorporated into the polymer to enhance the selectivity for CO2 sensing. Unfortunately, the siloxane groups resulted in too low stiffness polymers be usefull for application in the FBG chemical sensor. In fig. 8 an example is given for a coated FBG with a 25 µm layer of the PI-II polymer, exposed at three different temperatures (25, 50 and 120 °C) and five different pressures (1, 10, 50, 100 and 200 bar).
PI-V
B. Gas exposure tests
Cross sensitivity experiments have been performed by measuring the response of the sensors against nitrogen, hydrogen sulfide and methane at ambient conditions (23 °C and 1 bar gas). Fig. 9 shows the ∆λ values for 25 µm thick coated fibers.
∆λ (pm)
The exposure experiment were also performed at elevated temperature. It was found that at higher temperature the water absorption is lower and the optical signal is also lower, but still in good agreement with the calculations from the toolbox.
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