540
111.
Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 540-545
Symposium on New Concepts in Applied Polymer Science R. H. Mumma, Chairman 183rd National Meeting of the American Chemical Society Las Vegas, Nevada, March 1982
Evaluation of Environmental Effects on Candidate Polymeric Materials for Underwater Optoacoustic Sensors Rodger N. Capps,’ Ira J. Bush, Steven T. Lleberman, and Susan E. Eveland Naval Research Laboratory, Underwater Sound Reference Detachment, Orlando, Florida 32856
A number of commerically available polyurethane encapsulants and two optical fiber coating materials have been investigated for use in underwater optoacoustic sensors. The long-term compatibility of optical fiber jacketing materials and encapsulants with several currently used acoustic coupling fluids and water has been investigated. Changes in the properties of coating and encapsulant materials as a function of immersion times and temperatures were monitored. The effect of these changes on the acoustic sensitivity of optical fibers was evaluated. Dynamic mechanical properties were measured for the materials under consideration. Based upon the results of the evaluation, the suitability of these materials for application in underwater optoacoustic sensors is discussed.
Introduction Since the first demonstrations (Bucaro et al., 1977; Cole et al., 1977) that optical fiber interferometers could be used as acoustic sensors, there has been a good deal of interest (Bucaro and Carome, 1978; Carome et al., 1977; Culshaw et al., 1977; Shajenko et al., 1978; Price, 1979; Layton and Bucaro, 1979) in the utilization of optoacoustic interactions in glass fibers to detect acoustic waves. A practical fiber-optic hydrophone would alleviate a number of problems commonly encountered with conventional electroacoustic transducers, such as impedance matching, susceptibility to electromagnetic interference and thermal shock, and limited area coverage. Additionally, optoacoustic sensors are very sensitive detection devices. Considerable effort is now being expended to develop fielddeployable underwater acoustic sensors (Davis et al., 1980). A fiber-optic hydrophone deployed in an ocean environment requires an encapsulation system to protect the coating of the optical fiber from attack by acoustic coupling fluids and seawater as well as to mechanically fix the fiber in the desired geometry. The performance of fiber-optic hydrophones (particularly those based upon interferometric designs) is susceptible to factors such as hydrostatic pressure changes, temperature variations, and mechanical vibrations. Intelligent design and application require that an evaluation be made of the influence of these factors on hydrophone performance and life characteristics. An illustration of a simple fiber-optic acoustic interferometric sensor is shown in Figure 1. The principle of operation is based upon a general Mach-Zehnder interferometer (Hecht and Zajac, 1979). Light from a laser is beam split by BS 1 and coupled into single-mode fibers via proper focusing optics (generally microscope objectives This article not subject to
U S . Copyright.
for a laboratory setup). The upper leg is the signal leg and consists of a coil of fiber generally 10 to 100 m in length rolled up into a “sensing coil” so as to be compact enough to be much smaller in diameter than the acoustic wavelengths of interest (to be detected). The coil may be loosely wound or supported by some type of mandrel. The lower leg is the reference leg and is generally a piece of fiber as long as the sensor leg so as to match its optical path length. The beams are coupled out of the fibers, recollimated via the proper optics, and recombined with a beamsplitter, BS 2, to produce an interference pattern on the detector. The transduction mechanism utilizes acoustically induced phase modulation. The phase change of the optical beam traveling through the sensing fiber (with respect to the reference leg) is given by A$ = k(dn/dF’+ n/L dL/dP)PL
(1)
where n is the optical index of refraction, P is the acoustic pressure, L is the fiber length, and k is the optical wave number. The transduction effect is thus seen to be due to both a strain-induced index change and a pressure-induced length change. The sensitivity of the fibers used in such sensors will be affected by a number of factors (Jarzynski et al., 1981; Hughes and Jarzynski, 1980; Bucaro and Hickman, 1979; Tateda et al., 1980; Lagakos et al., 1981). These include the glass compressibility, the fiber coating thickness and geometry, the coating compressibility (inverse bulk modulus), the Young’s modulus of the fiber coating, and the boundary conditions with which the fiber is mounted with respect to the acoustic field. Therefore, a knowledge of the dynamic mechanical properties of the fiber coating, together with the coating thickness and geometry and the boundary conditions for exposure to the
Published 1982 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 541 ACOUSTIC KEDIUM
ACOUSTIC MEDIUM
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S I N G L E MODE
COUPLIN ?FIBER G
SINGLE,’ MODE F I B E R
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Figure 1. Mach-Zehnder interferometer.
fluids
coatings
encapsulants
1
FIBER COIL
-
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Table I. Materials for Compatibility Studies
TRANYER
r
Shell Sol 71 Hytrel 7246 PR 1574 Isopar M Tefzel 200 PR 1547 castor oil Dow Sylgard PR 1 5 9 2 Conathane EN-5 Conathane EN-9 Goodrich Castable Rho-C 10-7/16
acoustic field, is necessary in order to make theoretical predictions concerning the relative effects of different fiber jacketing materials on acoustic sensitivity. Knowledge of material properties is also a requisite for prediction of the performance of such sensors with various mandrel materials (Africk et al., 1981). The basic problem from a material standpoint is to provide a coating and encapsulation system that is chemically, mechanically, and acoustically compatible with the optical fiber element, acoustic coupling fluids and associated optical Connectors. In this work, we have evaluated several candidate polymeric materials and acoustic coupling fluids for potential application in fiber optic acoustic sensors. We have also characterized single mode fiber interferometric acoustic sensors as a function of hydrostatic pressure, temperature, and various environmental situations. Experimental Section The experimental approach taken was a threefold one. First, it was necessary to determine whether long-term exposure of the polymers to the solvents to be tested would cause severe degradation of the polymers. This was done by using standard ASTM procedures to monitor changes in weight, swell, and shore A hardness as a function of immersion time at various constant temperatures (ASTM D 471-79, ASTM D 2240-75). It was also necessary to determine whether any changes in sensitivity of the coated optical fibers could be detected after a known period of immersion in the various solvents at different temperatures and to correlate these changes (if any) with changes in the moduli of the coating material. Finally, the dynamic Young’s modulus and loss tangent of the materials under evaluation were measured over the frequency and temperature range of interest. The materials listed in Table I were chosen for evaluation. Shell Sol 71 and Isopar M are hydrocarbon solvents, manufactured by Shell and Exxon, respectively, that are often used as acoustic coupling fluids in sonar systems. Synthetic seawater and deionized water were also used as immersion solvents. All of the encapsulation systems chosen were liquid systems, to allow for ease of handling and application. (In the context of this paper, “coating” or “jacketing” may be taken to mean a material used to cover an optical fiber waveguide. An “encapsulant” may be taken to mean a material used to protect a coated fiber or to mechanically fix it in place.) There selection was based upon information available concerning their hy-
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drolytic stabilities and chemical properties (Lequin, 1969, 1973; Lequin and Weller, 1974; Weller and Lequin, 1977; Capps et al., 1981). The optical fiber used in the sensitivity measurements was ITT-T1601 single mode fiber with a composite coating of Dow Sylgard 184 and Hytrel7246. Sylgard 184 is a silicon-based polymer, while Hytrel is a thermoplastic polyester. At the time these tests were undertaken, T1601 fiber coated with Tefzel (a fluorocarbon polymer) was not commercially available. Bulk samples of Tefzel were evaluated for chemical compatibility and dependence of dynamic mechanical properties upon frequency and temperature. This was done to determine whether or not fibers coated with this material should be obtained for evaluation for use in underwater optoacoustic sensors, under a special arrangement with ITT. In order to avoid phase fluctuations due to changes in temperature, light intensity fluctuations, and rotation of polarization, the stabilized Mach-Zehnder interferometer shown in Figure 2 was used to measure the acoustically induced optical phase modulation (Bush, 1981a). Light from a single-frequency helium-neon laser is propagated into an electrooptic modulator (a Bragg diffractor) to produce two optical beams of different frequency spatially removed from each other which constitute both arms of the interferometer. Recombination of the beams produces a modulated carrier frequency where the acoustic information (and any other phase producing disturbances) forms the sidebands. The detected signal is fist processed for proper amplitude. The phase is then detected and synchronously tracked via feedback to a linear phase modulator in the reference leg. The phase modulator consists of a length of fiber wrapped tightly around a thin-walled piezoelectric cylinder (PZT-4). Its frequency response is flat below 40 kHz. The acoustic coupler used in these measurements (illustrated in Figure 3) has a uniform acoustic field up to a frequency of 1400 Hz with
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
542
I 1S92
Figure 4. Apparatus for Young's modulus measurements. 01
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Figure 5. Plot of fractional weight gain vs. immersion time for encapsulants and coating tested in deionized water. 01 IO
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Figure 8. Plot of fractional weight gain vs. immersion time for encapsulants and coating tested in Isopar M. PR 1574
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Figure 6. Plot of fractional weight gain vs. immersion time for encapsulants and coating tested in synthetic seawater.
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DB-grade castor oil as a fill-fluid. The theory of such couplers has been discussed by Bobber (1969). The complete details of the experimental system have been described elsewhere (Bush, 1981a,b). The apparatus used for dynamic Young's modulus measurements is shown in Figure 4. The theory of measurement is based upon the technique described by Norris and Young (1970). A harmonic displacement is applied to one end of a sample bar and the ratio of end accelerations of the sample is measured. The technique accounts for end-mass effects of an accelerometer. The equations of motion within the sample are considered in terms of displacement, strain, and stress. From the solution of these equations, the complex modulus and the loss tangent of the material may be obtained. The equations of motion
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Figure 9. Plot of fractional weight gain vs. immersion time for encapsulants and coating tested in Shell Sol 71.
within the sample are most easily solved at resonance (a 90' phase shift). Data acquisition is automated with an Apple I1 Plus computer by using IEEE-488 bus instrumentation. After observation of the initial wave forms on an oscilloscope, the beginning frequency is entered from the keyboard. The frequency synthesizer is programmed to step through the frequency range until a resonance is found. A t resonance, a voltmeter is used to measure the output ratio of the two accelerometers. The Young's modulus and loss tangent are then calculated from the solution of the equations of displacement within the sam-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 543
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Figure 10. Plot of acoustic sensitivity vs. frequency at various hydrostatic pressures and 20 O C .
ple. Since subsequent resonances are nearly overtones of the first, it is a simple matter to measure the first five or six modes automatically. The data are then plotted as Young's modulus and loss tangent vs. frequency. Results and Discussion Plots of fractional weight gain versus immersion time in all five solvents are shown for several of the materials tested in Figures 5-9. Not all of the materials tested are shown, to avoid obscuring the data. Hytrel and Tefzel showed little change in properties after 40 days immersion at 75 OC in the five solvents. Hardness changes for Hytrel and Tefzel could not be measured with the available experimental equipment. No weight change plots for Tefzel are shown, since the fractional changes in this material were on the order of the experimental uncertainties in the measurements. Of the encapsulation compounds tested, PR 1592 exhibited the best overall behavior in the acoustic coupling fluids in terms of fractional weight gain, swelling, and softening. Shell Sol 71 and Isopar M caused severe degradation of some of the compounds tested. Goodrich castable Rho-C, in particular, is unsuited for immersion in these compounds. It does appear to be suitable for use with castor oil. A decision as to the suitability of a given elastomer-fluid combination is a subjective judgment. The most frequently used criteria (ASTM D 471-79; ASTM D 2240-75; Timme, 1975; Baker and Thompson, 1979) are changes in the weight, hardness, or dimensions of the elastomer. In general, small weight changes, lack of softening, and lack of swelling are fairly good qualitative indicators as to the suitability for use of a given material in a particular situation. Weight change of the solid upon prolonged immersion is an imperfect test, since it does not directly simulate any end use. Additionally, some elastomers may exhibit complex weight-change behavior with time, indicating that several different processes may be occurring simultaneously. In general, swelling, chemical reaction between the polymer and solvent, and permeation of the solvent through the polymer may all take place. It is possible (and usually probable) that all of these will occur simultaneously. In cases where the major process taking place is a simple diffusion of solvent through the polymer, a log-log plot of fractional weight change vs. time will give a straight line with a slope of one-half (Timme, 1975; Baker and Thompson, 1979). Results where this is not the case indicate that swelling and/or chemical reactions, such as bond breaking or leaching of plasticizers, are occurring. The latter behavior was observed for most of the materials tested. The behavior of PR-1592 in castor oil exactly fitted
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a simple diffusion-controlled process. A calibration curve of optoacoustic sensitivity (optical phase shift normalized to the acoustic pressure level inside the coupler) vs. frequency for unsoaked T1601 fiber at a measurement temperature of 20 OC is shown in Figure 10. The dependence of sensitivity upon hydrostatic pressure is negligible, being within the range of experimental error (about 3%) of the measurements. This indicates that the elastic properties of Hytrel are relatively insensitive to hydrostatic pressure. Comparison of Figure 10 with Figure 11reveals a frequency dependence of sensitivity, consistent with the slight frequency dependence of the Young's mo-
Ind. Eng.
544
Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
UNSOAKfD
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ea
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Figure 14. Frequency dependence of the Young's modulus of Tefzel 200 for three temperatures.
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Figure 16. Young's modulus vs. frequency for Hytrel7246 a t 23.4 "C after 45 days immersion in various solvents at 75 "C.
Jarzynski, 1980; Lagakos et al., 1981; Budiansky et al., 1979) as (2) &/4J = - n2[(P1, + PI&, + P1,€,1/2 where Pll and P12are the Pockels coefficients of the core, €2
e, is the fiber core axial strain, er is the radial strain, and n is the refractive index of the core. The first term of eq 2 represents the phase change due to the fiber length change, while the second and third terms are due to the refractive index modulation of the core. Lagakos et al. (1981) have studied the pressure sensitivity of optical fibers as a function of coating thickness and Young's and bulk moduli of the coating material. cnsrbu OIL Their results showed that, in the limiting case of a very thick coating, the fiber sensitivity approaches a limit that L A 1 5 0 !- _ _ _ __ is independent of the Young's modulus, with the fiber 0 0 50 I oa 150 FREOUENCY (kHz1 sensitivity depending heavily upon the coating compresFigure 15. Acoustic sensitivity vs. frequency for fibers after 45 days sibility (inverse bulk modulus). immersion in various solvents a t 75 "C. For fibers with typical coating thicknesses, the sensitivity will be a function of both the bulk modulus and the dulus of Hytrel. Figure 12 shows the temperature deYoung's modulus (Lagakos et al., 1981) of the coating. For pendence of the sensitivity of Hytrel-coated fiber at a a material with a high Young's modulus, only a slight reference hydrostatic pressure of 1000 psi. The observed coating thickness will be required to reach the thick coating behavior is consistent with the temperature dependence case in which the sensitivity is determined mainly by the of the Young's modulus of Hytrel, shown in Figure 13. bulk modulus. For a typical coating thickness, maximum Comparison of Figure 13 with Figure 14 shows that the enhancement of sensitivity would require that the coating modulus of Tefzel is less temperature sensitive than that material have both a high Young's modulus and a low bulk of Hytrel, indicating that the sensitivity of Tefzel-coated modulus. fiber would be less affected by temperature changes than The present results are understandable in light of the Hytrel-coated fiber. Figure 15 shows the acoustic sensistudy of Lagakos et al. (1981). For the Hytrel-coated fiber tivities of various fibers a t a measurement temperature of used here, the sensitivity will depend upon both the bulk 20 "C after 45 days immersion in four different solvents modulus and the Young's modulus, with the bulk modulus a t 75 "C. In all cases, it can be seen that there is an being the more important factor. It has also been shown increase in sensitivity relative to the unsoaked fiber. (Jarzynski et al., 1981)that the sensitivity of this particular Castor oil and Shell Sol 71 showed less effect on sensitivity coating is due to the outer jacket of Hytrel and is influthan water or Isopar M. Figure 16 illustrates the change enced very little by the inner jacket of Sylgard. The in Young's modulus of Hytrel after 45 days immersion in Young's modulus measurements for Hytrel reveal a dethe four solvents. The general trend revealed by a comcrease after a period of solvent immersion. In view of the parison of Figure 15 with Figure 16 is an increase in senresults of the solvent immersion tests, it is reasonable to sitivity as the modulus of the jacketing material decreases. expect that a decrease in the bulk modulus is also taking The modulus measurements of the Hytrel soaked in Shell place. In this case, one would expect an enhancement in Sol 71 are considered to be less reliable than the other acoustic sensitivity relative to the unsoaked fiber. This measurements, due to difficulty in making the sample is what our measurements reveal. adhere to the shaker table. Conclusion The phase retardation of light in an optical fiber, inThe present results indicate that Hytrel and PR 1592 duced by a pressure change AP,is defined as A#/AP. This phase retardation is due mainly to the change in the index would both be suitable materials, in terms of swelling, softening, and weight gain, for use in an underwater opof refraction of the glass fiber and the change in the length toacoustic sensor system. PR 1592 appears to be best of the fiber. For the plane-strain model, the induced suited for long-term exposure to Isopar M and Shell Sol fractional phase change may be expressed (Hughes and ~
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 545
71. It also has a hydrolytic stability (projected to be in excess of 50 years at 23 "C) suitable for use in naval ordnance (Weller and Lequin, 1977). It has also been shown that long-term exposure of Hytrel-coated fiber to water and acoustic coupling fluids will cause a change in the sensitivity of the fiber. The changes in sensitivity are attributed to changes in the elastic properties of the jacketing material. Shell Sol 71 has the least effect on sensitivity. On the basis of both the immersion tests and the modulus measurements, Tefzel appears to be a better candidate than Hytrel in terms of environmental resistance and lack of temperature effects on sensitivity. The present measurements were made on fibers which were not wrapped on mandrels. Complete characterization of materials for systems use will require that fabrication and testing of model elements incorporating the variables of fiber coating and encapsulant materials, mandrel materials, and bend radius be undertaken. Other materials, such as nylon, polypropylene,and Tefzel, should be tested as optical fiber coatings. I t will also be necessary to determine whether the encapsulating material will have any effect on sensitivity. It is important to optimize the sensitivity of the fiber with respect to the coating for use in the actual sensor and to desensitize it with respect to coating materials (such as metals) that are used as cable leads. The properties of each new fiber and coating material will most likely be different and must be evaluated before a selection of the optimal combination can be made. The results of the present investigation indicate that this can be done in a systematic manner. Acknowledgment This research was supported by the Fiber Optic Sensor System Program of the Naval Research Laboratory. The assistance of Lisa Fagerstrom in carrying out preliminary experiments is gratefully acknowledged. Literature Cited Africk, S.; Burton, T.; Jameson, P.; Ordubadl, A. "Design Studies for Flber Optic Hydrophones"; Bok, Beranek, and Newman, Inc. Inc. Report No. 4658, Cambridge, MA, Aug 1961. ASTM D 471-79, "Standard Test Method for Rubber Property-Effect of Liquids"; Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, 1980; Part 37, p 103. ASTM D 2240-75, "Standard Test Method for Rubber Property-Durometer Hardness"; Annual Book of ASTM Standards; American Society for Testing Materials: Philadelphia, PA, 1980; Part 37, p 576. Baker, G. R.; Thompson, C. M. "The Effect of Seawater on Polymers"; NRL Memorandum Report 4097, Underwater Sound Reference Detachment,
Naval Research Laboratory: Orlando, FL, Nov 14, 1979. Bobber, R. J. "Underwater Electroacoustic Measurements"; Underwater Sound Reference Division, Naval Research Laboratory: Orlando, FL, 1969; Chapter 2. Bucaro, J. A.; Carome, E. F. Appl. Opt. 1978, 77, 330. Bucaro, J. A.; Dardy, H. D.; Carome, J. A. J. Acoust. SOC.Am. 1977, 62, 1302. Bucaro, J. A.; Hickman, T. R. Appl. Opt. 1979, 78, 938. Budiansky, B.; Drucker, D. C.; Kino, G. S.;Rice, R . M. Appl. Opt. 1979, 18. 4085. Bush, I. J. Master's Thesis, Unlversity of Central Florida, Orlando, FL. Dec 1981a. Bush, I. J. "Technical Digest, Proceedings of the Conference on Lasers and Electrooptics"; Washington, DC, June 1981b, p 100. Capps, R. N.; Thompson, C. M.; Weber, F. J. "Handbook of Sonar Transducer Passive Materials"; NRL Memorandum Report 431 1, Underwater Sound Reference Detachment, Naval Research Laboratory: Orlando, FL, Oct 30, 1981. Carome, J. A.; Dardy, H. D.; Carome, E. F. Appl. Opt. 1977, 76,330. Cole, J. H.; Johnson, R. L.; Bhuta, P. G. J. Acoust. SOC. Am. 1977, 62, 1136. Culshaw, B.; Davies, D. E. N.; Kingsley, S. A. Electron. Left. 1977, 13, 760. Davis, C. M.; Einzig, J. A.; Bucaro, J. A.; Giallorenzi, T. G. "Fiber Optic Sensor System (FOSS) Technology Assessment"; Dynamic Systems Inc. Report SDI-TR-80-01, McLean, VA. Jan 1, 1980. Hecht, E.; Zajac, A. "Optics"; Addison Wesley Publishing Co.: Reading, MA, 1979: 0r 290. ~ - Hughes, R.; Jarzynski, J. Appl. Opt. 1980, 79,1799. Jarzynski, J.; Hughes, R.; Hickman, T.; Bucaro, J. J. Acoust. SOC.Am. 1981. 69. 1799. Lagakos, N.;'Schnaus, E. U.; Cole, J. H.; Jarzynski. J.; Bucaro, J. A. I€€€ J. Quant. Nectron. 1981, OE 18, 683. Layton, M. R.; Bucaro, J. A. Appl. Opt. 1979, 78, 666. Lequin, D. S. "Epoxy Resin Potting Compounds for Ordnance"; Naval Ordnance Laboratory Technical Report 69-1 74, Naval Ordnance Laboratory: Whlte Oak, Silver Spring, MD, Nov 24, 1969. Lequin, D. S. "Room Temperature Curing Epoxy Resin Potting Compounds for Ordnance"; Naval Ordnance Laboratory Technical Report 73-36, Naval Ordnance Laboratory: White Oak, Silver Spring, MD, July 17, 1973. Lequin, D. S.;Weller, R. D. "Hydrolltlc Stability of Polyurethane and Epoxy Encapsulants"; Naval Ordnance Laboratory Technical Report 74-82, Naval Ordnance Laboratory: White Oak, Silver Spring, MD, May 17, 1974. Norris, D. M., Jr.; Young, W. C. "Longltudlnal Forced Vibration of Viscoelastic Bars with End Mass"; Cold Regions Research and Engineering Laboratory Special Report 135, Hanover, NH, Apr 1970. Price, H. L. J. Acoust. SOC.Am. 1979, 66,976. Shajenko, D.; Flatley, J. P.; Moffet, M. B. J. Acoust. SOC. Am. 1978, 64, 1286. Tateda, M.; Tanaka, S.; Sugarawara, Y. Appl. Opt. 1980, 19,770. Timme, R. W. "Polyalkylene Glycol as a Transducer Fluid"; NRL Memorandum Report 3146, Underwater Sound Reference Detachment, Naval Research Laboratory: Orlando, FL, Oct 1975. Weller, R. D.; Lequin, D. D. "Hydroliiic Stability of Some Commercially Available Non-MOCA Cured Polyurethane Encapsulants"; Naval Surface Weapons Center Technical Report, Naval Surface Weapons Center: White Oak, Sliver Spring, MD, Dec 8, 1977.
-.
Received f o r review May 20, 1982 Accepted July 26, 1982 This material was presented in the Division of Organic Coating and Plastics Chemistry at the 183rd National Meeting of the American Chemical Society, Las Vegas, NV, Apr 1, 1982.
