PROCEEDINGS OF SYMPOL-2013

Urethane Potting of Acoustic Transducers for Acoustic Travel-Time Current Meters

Albert J. Williams 3rd and Fredrik T. Thwaites

Woods Hole Oceanographic Institution Woods Hole, MA 02543, USA

awilliams@whoi.edu

Abstract— Piezo-ceramic acoustic transducers are electrically excited to cause dimensional changes that can generate a pressure/displacement wave. This is the transmit function. Changes in strain in the piezo-ceramic transducer generate a voltage and such changes in strain are the result of acoustic pressure/displacement stress on the transducer making it a receiver. To work in transmit and receive the electrodes of the transducer need to be electrically insulated yet pressure exposed. Immersion in an insulating fluid such as oil (castor oil is often used) can insulate yet expose the transducer to pressure but a window between the oil and the water (assuming the transducer is to work submerged in water) is still needed. Immersion of the transducer in an insulating solid solves the window problem. And there are two common materials that provide this insulating solid: epoxy and polyurethane. This paper will enumerate the experiences these authors have had with these two plastics but especially urethane.

Index Terms—Urethane encapsulants; transducer potting; ultrasonic acoustic transducers; epoxy encapsulants; acoustic transducers for acoustic travel-time current meters.

1. Introduction

To encapsulate a transducer in a plastic, the encapsulant must adhere to the electrodes of the transducer and even more importantly must adhere to the other materials in the transducer mount and the cable covering of the exciting electrical conductors. This need to adhere is a critical one that preoccupies a transducer potter

(one who encapsulates). Often a primer between the substrate and the plastic is needed. The problem becomes more severe when the water in which the encapsulated transducer is immersed is salt and still more severe if the pressure is high or temperature changes cause differential motion between the substrate and the encapsulants and stress the bond region. It is also possible that during the cure of the

Williams and Thwaites: Urethane Potting of Acoustic Transducers for Acoustic Travel-Time Current Meters

plastic there is shrinkage that introduces a pre-stress in the bonds.

2. Materials

2.1 Epoxy Epoxy is relatively easy to mix, degas, and,

when low-viscosity, to pour or inject. Epoxy is less attenuating of high-frequency sound than urethane and this often decides the selection of epoxy over urethane. In addition, epoxy adheres well to many substrates without special primers. However, epoxy shrinks with some exceptions during its cure. This shrinkage is often a serious problem. Two more serious objections to epoxy are water absorption and micro cracking. It is not certain how much water absorption really matters since the water is not a bulk conductor of electricity in this almost molecular distribution within the epoxy matrix. But, the micro cracking does lead to electrical leakage, which may eventually attenuate the electrical drive when transmitting and the electrical signal when receiving. It can also invalidate assurances of electrical isolation in a cabled observatory where ground faults are intolerable.

2.2 Urethane

Urethanes vary greatly in their properties with regard to sound attenuation, adhesion to substrates, water resistance (especially at the bond), viscosity, and pot life. High viscosity and short pot life make degassing difficult since the part A must be intimately mixed with the part B, a process that introduces bubbles. The bubbles are removed by evacuation but with high viscosity, the bubbles form, rise, and break slowly while the polymerization continues and further increases the viscosity. Then the viscous mix is placed in an injector and forced into the cavity that contains the transducers but if the viscosity has become too

great, the filling stops before the cavity is filled. The cure temperature must also be low enough to not damage the piezoelectric properties of the transducer. But even when all of these characteristics are acceptable, it may be that attenuation at the frequency of interest denies use of a particular urethane despite other desirable properties.

The perfect urethane potting material would mix and degas easily, inject well within the pot life of the curing urethane, not attenuate the sound at the frequency of interest, and adhere and remain attached to the substrates during prolonged submergence in salt water at depth. The search for this material continues. But lessons have been learned along the way that will be shared.

2.2.1 Environmental Constraints In the United States there are additional

constraints that make it more difficult to find (and keep) the perfect urethane. Some of those most closely matching the ideal have been taken out of production because mercury was used in one of the precursor compounds and others have been removed because of toxicity of uncured reactants. Then again, the business aspect of plastics production have caused a successful urethane to be discontinued when the manufacturer was acquired by another company who judged the market for this nearly ideal product to be unprofitable. Oceanic acoustic transducers are a niche too minor to support the research on primers and urethanes we require to solve our needs.

3. History

3.1 Conap TU-79 In 1979, when Williams first required

underwater acoustic transducers, Conap made a urethane, TU-79 [1], which met some of the ideal criteria. It mixed and degassed fairly

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easily and injected into molds with only moderate difficulty. It did not, however, continue to adhere to substrates at high pressure in sea water. This was strange because tests showed successful adhesion to epoxy, stainless steel, aluminum, other urethanes, and enameled copper wire during exhaustive tests in an oil-filled pressure chamber without primers or even cleaning. The only exception to good adhesion was that a coating of silicone grease prevented adhesion of the TU-79 to urethane tubing. It was later explained to us by Dr. Alphonsus V. Pocius from the 3M Company [2] that the problem with adhesion that had not been revealed in the oil filled pressure chamber was that the adhesion was probably simple hydrogen bonding, not chemical bonding, and water destroyed the hydrogen bond. He called the test for hydrogen bonding the Spit Test in which bonded pieces were pulled and the bond region spit upon. If the bond were merely hydrogen bonding, the parts would fly apart.

3.2 Fluid Polymers PF5-7B

Before the cause of the de bonding of TU-79 had been explained, TU-79 had been taken off the market and was eventually replaced by TU-79MF, for mercury free, an unsatisfactory replacement for our purposes. Our search for a replacement found PF5-7B by Fluid Polymers, later acquired by Lord Chemical. This was more difficult to mix and degas but it did not exhibit de bonding at high pressure exposure in sea water. Dr. Pocius noted that it contained polyether polyol [3], an extremely hydrophobic component, so that while still probably depending on hydrogen bonds for adhesion, the bond stayed dry [4]. This became a critical chemical component we sought in subsequent selections of urethanes when PF5-7B was discontinued.. It had been valuable to us as a potting compound since it adhered well to other urethanes, stainless steel, aluminum, PVC and

ABS plastic, as well as the epoxy with which the transducers were covered. The acoustic properties of PF5-7B were not tested since epoxy covered the transducers, not PF5-7B. But its valued hydrophobic property was sought in its replacement.

3.3 Conap EN4 – Acoustic Properties

While the acoustic properties of PF5-7B were never tested, the acoustic properties of other urethanes were of interest for Williams's first transducer designs and, under the mistaken belief that urethanes chemically bonded to one another if they were clean enough, he used the Conap urethane UC32 for the structure holding the transducer since UC32 became very hard, simulating an injection molding plastic, and covered it with Conap EN4 [5], selected because it had the same ρC (density times speed of sound) as sea water. This encapsulating compound, EN4, had minimal acoustic attenuation at 2MHz. The bond to the UC32 structure was difficult to ensure and eventually the procedure was to temporarily cover the exposed transducer with a paper mask and sand blast the surrounding UC32 with clean grit and dry nitrogen in the sandblaster. The EN4 was poured on top after the mask was removed and mounded slightly over the center of the transducer but this was of no concern since with the ρC match to seawater there was minimal reflection and the focal effect was close enough so that the acoustic beam remained the 5° calculated from the diameter of the transducer and its resonant frequency.

Epoxy, as used later on transducer coverings, does not have attenuation issues but does have electrical leakage issues after high pressure exposure to sea water. Efforts to seal the epoxy to minimize the electrical leakage have been only slightly successful. These attempts have been to spray or brush urethane enamel paint on the epoxy and to coat the entire

Williams and Thwaites: Urethane Potting of Acoustic Transducers for Acoustic Travel-Time Current Meters

transducer structure with a non-fouling silicone material [6]. The search for a very low viscosity urethane that can be injected into the ABS supporting-structure holding the transducers continues.

Meanwhile EN4 has been taken off the market due to environmental issues but EN7 [7] is still offered with its part A the same as part A for EN4 but a different part B. Its acoustic properties are not as good as EN4 and it is difficult to keep bubble free despite injecting and curing it at elevated pressure in a pressurized cooking pot.

3.4 Cytec EN1556

Cytec EN1556 [8] is the present replacement for Fluid Polymers PF5-7B and despite high viscosity it has been successful as a potting urethane. It contains polyether polyol so it has the hydrophobic qualities that protect the bonds that are probably simply hydrogen bonds to other urethanes and to primers. It has too high a viscosity to inject into the mold containing the transducers where it could replace the low-viscosity epoxy, which is subject to electrical leakage. In any case, its attenuation at 2MHz is not known.

3.5 Cytec TU-401/701

Cytec TU-401/701 [9] is easier to work with than EN1556 and is under consideration for filling the stainless steel tubes of the acoustic travel-time current meter since its viscosity is lower than EN1556. It remains to be seen if its adhesion properties are acceptable.

4. Acoustic Attenuation

4.1 Cytec TU-401/701 & EN7 In a new sensor design by Thwaites [10], the

transducer is mounted and encapsulated in a single urethane. With EN4 no longer available,

a replacement, Cytec TU-401/701, was used. This could be degassed and injected into molds adequately but attenuated the sound at 1.75MHz a factor of 10 over the epoxy used in similar transducers.

EN7 had lower attenuation but could not be injected bubble free. When the transducer molds were filled inside a pressure pot at 15 psig (1 atmosphere over pressure) some bubbles remained and only when the mold was filled in a pressure pot at 70 psig (4.5 atmospheres over pressure) was the urethane bubble free. But when heated for curing to 50C, bubbles were produced in the excess pour material in the filling funnel so the procedure , while adequate, was marginal. Even more damaging in the case of TU-401/701 was an increase in attenuation as the temperature was lowered, an effect not observed with other encapsulates.

EN7 continues to be used, and although bubbles remain a concern the attenuation has improved by a factor of three. This is the present status of our transducer potting.

5. Primers and Adhesion

Urethanes are desirable for potting due to their elasticity, meaning in this case low Young’s modulus stiffness and large elastic strain limits, which reduces stress on the bond. Such stress results from distortion due to pressure, temperature, and shrinkage during cure. However, the bond must be as strong as possible initially and remain strong during exposure to salt water even when distorted by environmental conditions. Primers are recommended by the manufacturer for many urethanes to aid bonding to various substrates; but such recommendations are rarely useful and in many cases are very poor indeed. We have routinely tested urethanes under consideration to replace what has been taken off the market by candidate replacements with primers recommended or useful in previous

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applications against many substrates we will be using and then immersing the tests pieces in salt water for weeks to months.

While the properties of particular primers are proprietary and somewhat mysterious, one class that has had the most consistent performance is Chemglaze 9944 [11], a pigment-containing catalyzed etchant resembling the no-longer-available AD-6 [12]. The authors assume that the pigment, being finely divided, offers a large effective surface area that permits the inherently weak bonds to the urethane a vast area upon which to act. In this way it achieves what might otherwise require a surface abrasion of the substrate to achieve such a large surface area.

A series of tests were performed and are summarized in Tables I to V below.

5.1 TU-401/701

Substrates as listed in Table I were coated with primer indicated in the second column in Table I and soaked in sea water in a bucket in the lab for a period of weeks initially and eventually for more than a month. Such adhesion tests are generally not done by the manufacturers of the urethanes of the primers. The adhesion was judged in a qualitative way by peeling a corner of the urethane and assessing the adhesion strength. A result of Excellent describes tight adhesion where the urethane eventually ripped before de bonding. Poor means that the urethane could be peeled off with little difficulty and fair indicates it took more force to peel the urethane off but it could be done.

Table I TU 401/701 Adhesion Tests

Substrate Primer Results

Aluminum Chemglaze 9944

Excellent

304 Stainless Steel

Chemglaze 9944

Excellent

Titanium Chemglaze 9944

Excellent

ABS Plastic Bare Poor

ABS Plastic [13]

AD-1161 [14]

Good

ABS Plastic PR-1167 [15] Fair to Poor

W-19 Epoxy [16]

Bare Good to Excellent

5.2 UC-54 For the next set of tests, a UC-54 urethane

was tested on substrates and primers indicated in Table II with the results indicated. There were several combinations that had good to fair bonding but the urethane itself failed in a brittle fashion, and cracked when pulled.

Table II UC-54 Adhesion Tests

Substrate Primer Results

Aluminum Chemglaze 9944

Fair to Poor

304 Stainless Steel

Chemglaze 9944

Poor

Titanium Chemglaze 9944

Poor

ABS Plastic Bare Good, but failure was brittle

Williams and Thwaites: Urethane Potting of Acoustic Transducers for Acoustic Travel-Time Current Meters

ABS Plastic AD-1161 Good, but failure was brittle

ABS Plastic PR-1167 Fair, but failure was brittle

5.3 ResinLab EP1056LV For the next set of tests, a ResinLab

EP1056LV urethane was tested on substrates and primers indicated in Table III with the results indicated. Like UC-54, this urethane had good to fair bonding for several combinations but the urethane itself failed in a brittle fashion, meaning that the urethane cracked when pulled.

Table III ResinLab EP1056LV Adhesion Tests

Substrate Primer Results

Aluminum Bare Poor

Aluminum Chemglaze 9944

Good, but failure was

brittle

Stainless Steel

Bare Fair, but failure was

brittle

Stainless Steel

Chemglaze 9944

Fair, but failure was

brittle

Titanium Bare Poor

Titanium Chemglaze 9944

Good, but failure was

brittle

5.4 TU-8080 TU-8080, supposed to be a replacement for

TU-79MF and TU-79, was tested as in the previous cases with the substrates and primers shown in Table IV. The results varied from Good to Worthless, meaning that some bonds were promising, although none were Excellent. But some bonds behaved like mold release with no adhesion at all and were judged Worthless. In one case the urethane separated from the substrate cleanly. The glue had failed.

Table IV TU-8080 Adhesion Tests

Substrate Primer Results

316 Stainless Steel

Bare Poor, material fails

316 Stainless Steel

AD-6 Good, material fails

Titanium Grade 2

Bare Worthless Poor

Titanium Grade 2

AD-6 Separated from

substrate, glue fails

Aluminum Bare Worthless

Aluminum AD-6 Good, glue fails

W-19 Bare Good, material fails

5.5 Masterbond EP30DP Masterbond EP30DP, stiffer than TU-8080,

was tested on the same substrates and primers as in Table IV with the results shown in Table V. The only cases where the bond could be

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judged as good, the material itself failed, meaning it cracked

Table V Masterbond EP30DP Adhesion Tests

Substrate Primer Results

316 Stainless Steel

Bare Material fails

316 Stainless Steel

AD-6 Material fails

Titanium Grade 2

Bare Poor

Titanium Grade 2

AD-6 separated

from titanium

Glue fails

Aluminum Bare Worthless

Aluminum AD-6 Glue fails

W-19 Bare Good, but material

fails

The tests samples of Table V were checked after two months of submersion to assess their hydrolytic stability. The only differences from the tests after a week of immersion were that the bonds failed before the material failed, probably indicating loss of bond strength rather than increase of material strength.

5.6 TU-79MF, EN7, EN9, EN11 A matrix of tests was made on stainless steel

with four weeks immersion in seawater shown in Table VI. Four urethanes were tested on the substrate wiped with alcohol and: left bare,

treated with Chemglaze 9944, PR-420, and FP-1306.

Table VI Matrix of TU-79MF, EN7, EN9, and EN11

with Bare Stainless Steel and three primers Bare Chemglaze

9944 PR-420

FP-1306

TU-79MF [17]

Worthless

Best Fair Good

EN7 Fair Fair Fair Worth-less

EN9 [18] Fair Poor Fair Worth-less

EN11 [19]

Fair Fair to good

Good Worth-less

6. Methods

Success with potting, particularly with urethanes, depends on procedures. Treatment of the raw components, preparation before mixing, mixing, preparations after mixing, injecting, curing, and demolding all require carefully developed procedures. Over the past thirty years, we have worked with about eight potters and observed carefully what might be causing problems, what worked best and experimented with techniques to improve results. A brief recounting of places where technique seems to matter should be beneficial.

6.1 Treatment of Raw Materials

Many components of urethanes react with water and it is necessary to keep them dry. In fact, when the can is first opened, air enters carrying with it moisture that eventually will react with the urethane to make a skin or even a hard surface that must be drilled through to get to the material beneath. Standard procedure has been to keep humidity low in the potting shop and it has long been noticed that things work much better in winter, during the heating season in New England, when the air is dry.

Williams and Thwaites: Urethane Potting of Acoustic Transducers for Acoustic Travel-Time Current Meters

Air conditioning was installed by necessity despite temperatures rarely requiring it during most of the year to extend the good urethane potting season. Finally, dry nitrogen is routinely sprayed into the partially used can before the lid is replaced. We have avoided refrigerating the urethanes, which might have increased their shelf life, for two reasons: the cold material is more likely to capture moisture when the can is opened, and many of the urethane raw components crystalize if cooled and need to be heated to dissolve the crystals.

6.2 Preparation before Mixing In a beaker, five or ten times the volume

wanted for the urethane, the part A is poured after getting the empty tare weight of the beaker. This gives the weight of part A material. The desired weight of part B is calculated from the instructions for the urethane and the composite weight of part A and part B is noted. Somewhat more of part B than required is poured into a second beaker, larger than required, and the two beakers, covered with paper towels, are placed in a vacuum chamber and evacuated. This removes the dissolved gases and pumping to about 1mm Hg pressure is continued until the bubbles collapse. Pumping is stopped and air is gently readmitted so the beakers can be removed.

The beaker with part A is placed on a balance with the weight set to just under the intended composite A+B weight plus tare of the beaker. Part B is poured into the beaker with part A until the balance tips and then, with a spatula, small amounts of part B are added until the final composite weight is reached. The time is noted. The spatula is wiped clean and the excess part B is returned to the can with dry nitrogen added. Note that if one of the components is toxic, it is this component that fills the role of part A so that when fully mixed, there is no unreacted residue of the toxic component remaining exposed in the potting shop.

6.3 Mixing A square ended metal spatula is used to mix

the part A and part B in the beaker, scraping the bottom, the sides, wiping the spatula on the rim of the beaker, and repeating for five minutes. No attempt is made to prevent air from becoming entrained since it is complete mixing that is more important. One must ensure that less than 1mm of separation can remain between streaks of part A and part B anywhere in the beaker. There is generally enough color difference that one can assess complete mixing by the uniformity of the mixed product. Thorough mixing has been necessary for success in potting and failures in some cases can be traced to inadequate mixing. Stirring is both circular and figure eight shaped crossing the center of the bottom of the beaker.

6.4 Preparation after Mixing The beaker is put into the vacuum chamber

and pumped down to 1mm Hg of pressure, being cautious that the foaming mixture does not overflow. Entrained air is removed but in addition there is evolution of gas from early stages of the reaction in many cases that also needs to be removed. Here is where experimentation can improve success since a nominal 10 minutes of degassing may be too little (resulting in bubbles) or too much (resulting in excess viscosity for the next step).

Prefilled syringes and disposable syringes for premixing volumes of urethane in ratios of 1:1, 2:1, 3:1 and 4:1 have been used with a 12 stage or 16 stage mixing nozzle to cut short some of these stages. The mixing nozzles split incoming fluid from the syringes into two paths and combine and split them again repeatedly. However we have found that it is still necessary to mix the output of the mixing nozzle in a beaker with a metal spatula and degas it.

In either case the mixed product is poured into a syringe and briefly degassed again.

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6.5 Injecting The injecting syringe is closed while

placing a short length of solid copper wire from the open end of the syringe to the urethane inside to allow air to escape as the plunger is inserted. When urethane appears at the wire, the wire is withdrawn. The nozzle of the syringe is inserted in a filling hole in the mold or the structure to be filled and heavy force is applied to cause the viscous urethane to move into the target. With several of the urethanes we have had to adapt to since our preferred urethane has been discontinued, we have had to use a pneumatically powered syringe to move the very viscous urethane rapidly enough to fill the target void before even higher viscosity developed. When the mold or structure is overfilled by about 10% (a length of clear tubing is extended above the top of the mold or structure to be filled) the syringe is removed and the filling hole sealed with a turn of Scotch Double Stick tape (sticky on both sides). The overfilling accommodates for both slight leakage and for rising bubbles of trapped gas.

6.6 Curing Room temperature cure for 24 hours

suffices for most of our urethanes but some require a week to achieve full hardness and some require elevated temperature after the first several hours to come to a full cure. At least one urethane with certain desirable properties has been rejected because it requires curing at 70C, a temperature compromising our piezo ceramic transducers.

6.7 Demolding There is no demolding for urethane filled

structures, such as the acoustic current meter sensor tube, but for molds, demolding is a critical step. Molds made of RTV, a silicone rubber, with no adhesion to urethane or epoxy, are often used. RTV is elastic enough to be peeled away from the injected urethane. The

RTV mold is cast around a pattern, which can be reused indefinitely while the RTV mold has a service life of about 10 uses, after which epoxy, especially, begins to stick and tear the RTV. For other urethane potting, aluminum molds, assembled from many parts, are disassembled and pried off the rather hard urethane injected into it. The surface of the aluminum is pretreated with a silicone spray for mold release and, after use, the mold is soaked briefly in acetone to remove traces of contamination by urethane and excess silicone.

When epoxy is to be demolded, heat is often helpful to give enough plasticity to the epoxy to allow it to deform and get free of the mold. Here is another place where experimentation is valuable to determine how long to allow the cure and how much heat to apply to achieve the desired result.

6.8 Summary The margin between success and failure is

thin in the case of urethane potting. There have been potters who seemed to do everything right but had low success while others without apparent effort got bubble-free urethane injections every time. Cleanliness is critical; humidity control can be important, and most important is the time spent in each stage of the process. Cutting one minute off the degassing step can make or break a pour or injection. The best potters experimented in minor ways to improve the procedure, almost to the point of superstition. Urethane potting is close to being an art.

7. Conclusions

While urethane manufacturers exhibit little interest in the need for hydrophobic, low acoustic attenuating, and low viscosity urethanes, there are other groups who are concerned. At OCEANS’2003 in San Diego, a presentation by scientists from NUWC [20] described their development of a urethane

Williams and Thwaites: Urethane Potting of Acoustic Transducers for Acoustic Travel-Time Current Meters

suitable for underwater acoustic transducers and also for underwater electrical connectors. They finished with a plea for a urethane manufacturer to produce this formula commercially. So far the closest approach has been 401/701, which, however, does not meet our low acoustic attenuation requirement at 2MHz.

The issue with primers remains as well. One of us, Thwaites, has tested available primers with each candidate potting urethane on substrates of ABS plastic, Emerson Cumings W19 epoxy, 316 stainless steel, 6061 aluminum, 6-4 titanium, and urethane tubing of unknown composition. In some cases a primer was recommended for our application by the urethane manufacturer. The substrates were cleaned, the primer applied as recommended, and the urethane coated on top. After curing as recommended for the urethane, the samples were immersed in buckets of sea water from the Woods Hole harbor. Every week a pull was made on a corner of the urethane and after two months the adhesion of the urethane was assessed. The results of several of these tests are shown in the tables. Clearly the recommendations do not offer a solution and testing in each case must be relied upon.

A new issue is being investigated by the team at NUSC (formerly NUWC). When a bond between urethane and a metal is subjected to a pH change from electrical current in sea water produced by a sacrificial anode, zinc for example, the bond fails from the exposed edge inward. It is not a bulk effect [21]. This imposes yet another constraint on urethane potting. It may have little impact on acoustic transducer potting but at some point there will be a urethane/metal bond and if the metal is cathodically protected, that bond will gradually fail. The solution may be to bury all such urethane/metal bonds beneath an O-ring to isolate the bond from sea water.

In conclusion, acoustic transducers for underwater communication, sonar, or current

measurement often rely upon urethane for potting of their electrical connections. The bonding, acoustic attenuation, and application constraints of mixing, degassing, and injecting into molds are issues that must be addressed. There is less interest and support from the urethane manufacturers than might be hoped for.

References

[1] http://plastics.ides.com/datasheet/e17128/conathane-tu-79, viewed 8/6/2013.

[2] Pocius, Alphonsus V., 1997. Adhesion and Adhesives Technology, Hanser Publishers, Cincinnati, 279 pp.

[3] http://www.ihs.com/products/chemical/planning/ceh/polyether-polyols-urethanes.aspx, viewed 8/6/2013.

[4] Personal communication.

[5] http://www.ellsworth.com/cytec-conathane-en-4-polyurethane-encapsulant-part-a-1-gal/, viewed 8/6/2013.

[6] Williams, A.J.3rd, “Sensor Fouling Prevention in an Acoustic Current Meter, MAVS”, Oceans 2012 Yeosu, IEEE Xplore 2012.

[7] http://prospector.ides.com/DataView.aspx?I=34&E=17094, viewed 8/6/2013.

[8] http://prospector.ides.com/default.aspx?A=AL&E=18933, viewed 8/6/2013.

[9] TU-401/701, datasheets no longer available, copies retained by the authors at Woods Hole Oceanographic Institution.

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[10] Thwaites, F.T. and R. Krishfield, “Development of a Second-Generation Ice Tethered Profiler with Velocity Sensor”. Oceans 2013 San Diego, IEEE Xplore 2013

[11] http://www.lord.com/products-and-solutions/coatings/product.xml/632, viewed 8/6/2013

[12] AD-6 formerly supplied by Conap is no longer available nor is there any record of it. It is said to be similar to Chemglaze 9944. Personal communication.

[13] http://en.wikipedia.org/wiki/Acrylonitrile_butadiene_styrene, viewed 8/6/2013.

[14] http://www.needfill.co.kr/cd/AD-1161.html, viewed 8/6/2013.

[15] http://www.needfill.co.kr/cd/PR-1167.html, viewed 8/6/2013.

16] http://www.ellsworth.com/emerson-and-cuming-stycast-w-19-epoxy-red-1-gal-pail/, viewed 8/6/2013.

[17] http://plastics.ides.com/datasheet/e31516/conathane-tu-79mf, viewed 8/6/2013.

[18] http://prospector.ides.com/default.aspx?A=AL&E=17097, viewed 8/6/2013.

[19] http://prospector.ides.com/DataView.aspx?I=34&TAB=DV_DS&E=17099&SKEY=34.1395901.64610540%3A9107ba28-42f4-441c-a69c-14c44ac9d0d2&CULTURE=en-US, viewed 8/6/2013.

[20] Ramotowski, T. and K. Jenne. NUWC XP-1 Polyurethane Urea: a New, “Acoustically Transparent” Encapsulant for Underwater Transducers and Hydrophones. Oceans 2003, pp. 227-230, IEEE Xplore (2003).

[21] Ramotowski, T., W.C. Tucker, and M.A. Rice. “Cathodic Debonding of Undersea Electronic Cable Connectors: Delamination Kinetics When Primers and Encapsulants Are Bonded Directly to Bare Metal Connector Backshells”. Oceans 2013 San Diego September 22-26, 2013. IEEE Xplore (2013).