Epoxy review

General Discussion
1

Based on my recent recent experience in the ocean environment where the tether potting is getting actual push,pull, and bend forces, I do not recommend using LocTite marine epoxy for potting, or structural attachments. It is brittle and lacks shock resistance. The potting in the tether thru connection is cracking (although not yet leaking). Because of this I ran some structural tests that indicated there is very little shear strength in an impact situation. Any attachments, such as thruster brackets, or access ports, made with this epoxy will fail if you impact a hard object in shear. For my tests, I attached a 2" ID, 2" long, PVC pipe stub to the plexiglass endplate on my ROV and left it there for several weeks, including ocean dives. When I noticed the cracking in the tether potting I tested the stub structural integrity by smacking it with a hammer (not that hard a smack, about what you would expect if your ROV did a free fall to the bottom where rocks are present). The epoxy joint instantly failed in the brittle fracture mode. There was no visible yield of the joint. Before you rely on this epoxy, I suggest you do your own testing.

Based on my personal experience, having built two wooden boats, I can recommend West Sytem G-flex epoxy. It is 1/2 the price per ounce of the Loctite, and is very well accepted in the boating community for underwater structural repair. The down side is the smallest amount you can purchase is 8 0z for 16-18 dollars. However, it is great to have around the shop for multiple uses.

Here is the marketing hype for it:
West System G-Flex Epoxy Resin

A toughened, versatile, liquid epoxy for permanent waterproof bonding of fiberglass, ceramics, metals, plastics, damp and difficult-to-bond woods. With a modulus of elasticity of 150,000 PSI, it is a bit more flexible than standard epoxies and polyester, but much stiffer than adhesive sealants. This gives G/flex the ability to make structural bonds that can absorb the stress of expansion, contraction, shock and vibration. It is ideal for bonding dissimilar materials. It can be modified with West System fillers and additives, and used to wet-out fiberglass tapes and fabrics. Mixed at a 1:1 ratio, G/flex gives you 45 minutes of working time at room temperature. It reaches an initial cure in 7 to 10 hrs and full cure in 24 hrs.

Sold in the following kits:

8 ounce kit containing 2 4 oz bottles each of epoxy and hardener

Just my opinion based on experience. Take it for what it’s worth.

 

Richard

2

@Richard,

Did you provide any kind of strain relief for the tether prior to it entering your cable penetrator?

I plan to anchor my tether to my ROV Chassis to take up the any kind of push-pull or bending that could otherwise compromise the potting at the cable penetrator.

Regards,

TCIII AVD

3

This is a standard Bisphenol-A (BPA) epoxy … frankly modulus of elasticity of 150,000 PSI can be considered low … and another BPA epoxy that I have used is rated at 533,000 PSI. All of that mumbo jumbo is going to boil down to how rigid the material is.

I tried to find the data for the Loctite Marine epoxy for that value and they don’t list it.

In regards to your joint failing … how did you prepare the surface of the joint? I have done yield testing with “potting materials” and depending on how you prepare the surface it can give way pretty darn quick or last to a how PSI yield.

In the guide that I wrote up, that is online here somewhere, about how to use the Loctite Marine epoxy or frankly any epoxy details some stuff that I do to help stuff “bond”.

Frankly a packing gland would better then all of the potting material on the market and at least you can replace the seals.

 

4

@TC3- See my picures on how the tether is secured at the penetration. I don’t care how much strain relief you think you are providing, that cable is moving around when your rov is in actual use. Bottom line: test it, before you use it on an expensive project. I feel it is too brittle for both potting and structural use, period. Just my opinion and the tests I base that on.

@ Harold- Standard epoxy joint prep of roughing and cleaning the surfaces.

Richard

5

Richard,

Thanks for sharing your test results!

I completely agree that the Loctite Marine Epoxy is fairly brittle. Despite that, it has held up really well for us, particularly in situations where there is a lot of structural integrity in the parts. For instance, in the Cable Penetrators, you might see a little cracking at the top, but within the penetrator the cable and epoxy is well supported by the aluminum and it is difficult to damage. That said I would not recommend using a potted penetrator to carry the full pull force of a tether. You should restrain the tether elsewhere using a cable thimble or other method. We never intended for the penetrators to be used like that and in the vast majority of situations, such as connecting a thruster to a watertight enclosure, there is very little strain on the cable.

Back in the day we tested lots of epoxies for the penetrators, including West Systems G/flex. It worked fairly well in a penetrator but the surface seems to crack open much more easily. We weren’t as confident with it as the Loctite epoxy.

I think G/flex is better suited to some applications, like attaching a thruster mount to the ROV, which requires strong shearing force. The Loctite is better for other applications, like filling the inside of the cable penetrators.

Since you brought this up and we want to be sure we are recommending the right product, we are going to re-test the G/flex epoxy on a few penetrators and see how it performs. If nothing else, it might be a good additional recommendation.

Thoughts?

-Rusty

6

@Rusty / Richard - One thing that you might try is keep the Loctite for the cable pentrator and use something like a chinese finger web to grip the cable and embed it in a spelter socket and use an epoxy like Socketfast.

The penetrator is to small for it to do anything but act as a seal. That is one of the reasons I was pestering you about your strain relief on the cable. I can’t remember exactly how you had it configured … but using a tow line independent of the actual data cable would be more ideal. Unless you want to invest some cash with a cable that is designed to be a tow member, use a line.

 

7

@Harold,

I second your observations concerning using just the Cat 5 cable to provide the vertical lifting of the WTC.

My approach would have been to use a nylon rope tow line tied to a hard point on the WTC and have the Cat 5 cable “float” along side the tow line with some kind of strain relief/stabilizer at the WTC prior to where the cable enters the cable penetrator to mitigate any external loads and keep the cable from swaying.

Also if Richard is using stranded Cat 5 then the vertical load of the WTC on the cable wire strands will begin to stretch the wires and could result in wire separation somewhere within the cable.

On my ROV Chassis I have mitigated unwanted power and control cable movement by securing the cables to the sides of the chassis with cable clamps and have also tie-wrapped the thinner control cables to their respective power cables which are thicker, relatively more rigid and tend not to have much movement.

Regards,

TCIII

8

I have just had a failure of the LocTite Marine epoxy joint used to attach the weight pods to the WTE. One of the four joints on one pod has cracked open longitudinally. These pods do take some abuse banging against the ocean bottom, structure and boat sides. However, none of the impacts have been greater than normal expected “wear and tear”. As I said before, this epoxy is too brittle and lacks impact resistance. I am redoing all of my attachment joints in West G-flex.

9

Richard,

Thanks for the update. I’m not surprised the Loctite failed for structural use. Regardless, I still think it works well for the cable penetrators.

I’m curious to see how the G/flex works out for the structure.

-Rusty

10

I’ll keep you posted on the structural performance of G/flex. My past experience in using it for bonding thermoplastic materials has been good.

11
Frankly a packing gland would better then all of the potting material on the market and at least you can replace the seals.
Agreed. Using a packing gland has many advantages over potting penetrators.

Has anyone done any additional testing of G-Flex versus Locktite for potting penetrators?

 

12

Well if everyone would like a packing gland … I can bang out a design pretty darn quick. We used packing glands on the spherical array for one of the classes of submarines. The amount was 1000+ so that is a lot of holes in the boat to leak. These things worked VERY well and never leaked.

Some of the things to consider with a gland is if you need anti-rotation follower to compress the grommet. It all depends on how you plan on using the gland.

Now the size is going to be bigger than the current penetrator but you can make the grommet to allow multiple cables to pass through etc.

 

13

OK, I know this is some serious thread archaeology but I’m curious if anyone ever followed up on using G-Flex epoxy and what the results were…

14

For ROV operations worldwide packing glands are only used on cables that are penetrating oil filled enclosures. The oil is kept at nominal 5 to 7 PSI above ambient pressure. Using packing glands instead of potted connectors may have some advantages but also some disadvantages. Loosening on packing nut, breakdown of gland material, difference in material properties of gland and cable jacket, physical properties including compression rating of cable jacket, etc. This is not a recommended solution when using multiple penetrators on an ROV system. It only takes one to leak to ruin your day.

15

Just to put my 0.05$ into this conversation:

We are using Loctite E-90FL, and we are so far very happy with the result. It seems to be slightly more flexible than other epoxies we have been trying, which is great for the tether.
You can see the TDS on here: