PTFE coating a fishing line for towing underwater probe in the Arctic

[MDLU]

Dear all,

- Our research team is looking to deploy an underwater towed probe (measure conductivity, temperature, depth for climate change research) in the Arctic (the towing boat in a previous mission,

https://www.nanuq2020.eu/).

- Probe is towed from the boat with a ~300m fishing line (at depths 0-100m).
- The line operates for continuous 12-hour-long daily missions over several months.
- Probe design highly dependent on fishing line: Power Pro 150lb is by far the best line we could find on the market for our design
- The line is a braid of Honeywell Spectra fibers (very well tested,

https://linghunt.com/101Spearfishin...-spectra-fiber-capability-guide-brochure.pdf)

- The Power Pro 150lb is 0.56mm in diameter

PROBLEM AND POTENTIAL SOLUTION
- Arctic water temperature is around -2C and the air temperature ~-15C. Line operates mostly underwater (-2C) but if towed from the boat, first few meters of the line will be exposed to air (-15C).
- We couldn't find any ice resistant line with the strength we need (>150lb breaking tension) and the PowerPro person suggested we look into PTFE coating the line to prevent water entering between fibers
(Power pro says, water gets between fibers and freezing creates small sharp particles that cut the fibers when the line bends under tension)

QUESTIONS FOR YOU

-> How feasible is for a university lab to coat the line (several hundred meters) with PTFE?
-> What parameters would you use (PTFE coating thickness), materials (primers, PTFE coating liquid/other), process (drying power/time)?

Thank you,
Dr. Matteo Di Luca

 

[3DDave]

I would say that coating with PTFE as you intend isn't going to work. It doesn't really adhere to anything - normally requiring melting to develop a mechanical grip or a chemical process to damage the PTFE to allow adhesives to stick.

The melting/fusion temperature for PTFE coatings appears to be in the 300-400C range. Spectra melts around 150C. Coated fiberglass rope and steel cable are available, but those materials have far higher melting points than PTFE does. One might consider a sort of flame spray, but that won't enter the fibers and any defect with that coating will just trap water. It will also trap air and cause the line to float.

I suspect an insulated sleeve from the reel to the water to protect the line from sudden freezing could be added, something like those hollow pool noodles, for example; add in some steel rings or a nylon tube sleeve to keep the line from cutting through the insulation. Use a heated housing for the reel to prevent freezing. This should prevent the ice from forming before it is wrapped around the reel.

Other coatings are also unlikely to work as Spectra is also very slippery and difficult to bond to. Tough task.

 

[MDLU]

Thank you 3DDave for your help,

Based on PTFE melting Temperature> spectra melting, melting PTFE on the fiber does not seem feasible.
However, PowerPro has a PTFE coated line (for operations in ice but too weak/small for us)
Based on this product, there should be a way to PTFE coat a line (doing it reliably maybe is in question?). I asked them if they could custom coat the 150lb line (no answer yet).

Thank you for the insulated sleeve idea, however, the probe needs to surface from time to time to acquire its own GPS coordinates (geotag data) and parts of the cable may be exposed to air (I didn't mention before to keep initial post short).
An alternative solution would be to keep the entire towing system underwater.

Thank you for mentioning steel: just checked piano wire may reach tensile strength levels close to the PowerPro 150lb (2.7GPa) and it could represent a valid alternative.

Best,
Matteo

 

[epoxybot]

You might want to look at silane coupling agents. They are low enough in viscosity to saturate the braid. hypothetically, the silane would create an air bubble around the line.

 

[3DDave]

PowerPro has a marketing department which has been careful to not specify what they mean by "coated."

 

[dik]

Silane or siloxane might be something to look into... and, I'm not aware of the chemistry with marine water, it may be possible to give added coatings after use. Drying it between, of course. I think the issue is to keep it from getting wetted to reduce drag from water adhesion forces.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[MedicineEng]

Well out of my league here, but it's an interesting question so I guess there is no harm for me to throw in my 5 cents of ignorance...

I think steel might be your best bet, even if you need to replace it due to salt water corrosion half way through the mission.
It might turn out to be a cheaper and more effective solution than a fancy and expensive coating that can get damaged when you are reeling in your probe and there goes the protection that you were trying to get.

 

[dik]

I would think you would want something that was hydrophobic... maybe steel because you might be able to use a thinner strand. I've not encountered any literature of the use of towing lines.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[MDLU]

Thank you epoxybot, 3DDave, dik and MedicineEng for your suggestions,

MedicineEng and dik, indeed, in terms of strength steel is very good (thin piano wires get up to 2.6-2.9GPa of tensile strength) and cheap enough for frequent replacements to avoid corrosion).
The problem is that the probe needs to be more buoyant to be able to resurface with the steel cable attached to it in case of cable failure near the boat.
That makes the probe aerodynamics more demanding (more elongated wings).

Overall, the "captain" of the mission is finally back from his sailing trip and it looks like he wants the line to be fully submerged (towing from below water surface)
to avoid floating algae and ice. It looks like we may not have an icing problem on the POWERPRO after all!

I'll keep you posted with the developments.

Thank you,
M