Kevlar/fireproof thread

[slothead]

Some months ago I described a test I did on the 1/4" Kevlar ribbon I got from Giant Leap. It refused to burn - thus getting my "seal of approval" as "fireproof". After I started constructing harnesses out of its wider brother (1/2" ribbon) and realizing that I would need to sew loops in the ends that would allow its connection to bulkheads, parachutes, etc. I decided to use Kevlar thread since Kevlar seemed to be so fire proof/resistant. I used something I got from a fly-tieing resource. It was very thin and very strong (the strongest thread I had ever found) so all was good in the harness department.

A while later it occurred to me that since I was using the Kevlar because of its fire-resistance near the ejection charges, that if the thread I was using was not as fire-resistance as the 1/2" ribbon, then the loops would fail and the harnesses would be useless. So I did some burn tests on the Kevlar thread I was using... The thread went up in a puff of smoke (in this test I used a wooden match as an ignition source)! Well that was a major disappointment.

I decided that maybe the fly-tieing "Kevlar" wasn't really Kevlar (even though it was obviously very strong). So I ordered another thicker Kevlar thread, this one from a maker of military fabrics and threads. I had to buy a pretty big spool of it, but I considered it part of an experiment, which if it worked I would have a lot of raw material, and if it did not, it would be a lesson learned.

This thread burned almost as fast as the fly-tieing stuff. It too was considered a failure. From that test I considered that maybe Kevlar thread didn't have the fire-resistant nature that its woven product did, so I tried something else. I ordered some Nomex thread. As you are probably aware, Nomex has as its primary characteristic fire-resistance as it is what is used to construct fire-resistant fabrics and clothing for the auto-racing industry, et al.

I got that tiny $10 spool of Nomex thread today in the mail. I immediately rant to the workshop to do a quick test. I didn't expect it to fare highly in this quick test, but I did expect it perform differently than the two Kevlar samples. That was not to be. The Nomex thread went up the same way as the Kevlar samples.

The bottom line, I must have the wrong mindset of the flammability of thread as compared to the flame-proof nature of the woven Kevlar ribbon or tubular construct.

So the question to you guys is: Am I going overboard worrying about the sewn loops in the harness and the thread used to sew them? Or is there a flame-proof coating I can cover the thread with to render them flame-proof too?

Tom

 

[cwbullet]

The correct term is flame resistant.

 

[Nathan]

I make my own flame resistant chute protectors by soaking cotton twill fabric in a solution of borax and boric acid. That works great but if you wash it you have to reapply.

 

[Eric]

Possibly with the thread out in open-air it is more likely to burn. I wonder if sewn tight against your harness you would have the same results. I often like putting a piece of heat shrink tubing over my sewn harness ends to protect the threads a little more. The thickness of the heat shrink will temporarily protect from the flash of heat.

 

[manixFan]

I think you need to test an actual article rather than the thread itself. Sew it up in a test harness and then apply flame against it. See if it fails the same way. My guess is with the surrounding material of the harness it will protect the thread and it won’t burn as it did before.


Tony

 

[slothead]

Good comments all! I might in fact risk the sacrifice of a Kevlar loop to test the "system" (I know the Kevlar will survive because that test is already completed).

While I was formulating a test plan, another advantage just occurred to me. When there is heat applied to the Kevlar/sewn loop (when the ejection charge first lights off), there is no load on it - and vice versa - when the Kevlar is out in the atmosphere (and loaded with rocket components), the thermal impacts are gone. That should be an additional saving characteristic.

Thanks guys

Tom

 

[Wallace]

I'd say for real world testing you'd almost have to have a sewn loop with heat shrink over it. How else would you keep the braided from nu-raveling? I've read many a time that glue and kevlar are a no go..

 

[dhbarr]

I have heard no epoxy or CA as they're entirely too rigid. I hadn't heard a blanket ban on all glues?

 

[Wallace]

Possibly? Never had the nerve to try any sort after all the hype....

 

[Bat-mite]

I use harnesses built by Ted Chernok of One Bad Hawk. He uses tubular Kevlar and Kevlar thread. I have never had the thread burn in one of his harnesses, and I am not good at protecting the harness (throw it in, only the chute goes in the Nomex blanket). I often have black harnesses when I pull them out, but the thread remains intact.

If you make your own loops, consider wrapping the threaded area in duct tape or electrical tape in it concerns you.

 

[sl98]

You might also want to consider your testing method. My thought is exposure directly to flame will give different results that exposure to an ejection charge. I would suggest doing some ground testing using a some spare body tube and BP.

 

[Bat-mite]

You might also want to consider your testing method. My thought is exposure directly to flame will give different results that exposure to an ejection charge. I would suggest doing some ground testing using a some spare body tube and BP.

Quite true. Rarely (maybe never?) would you have a flame with the intensity or duration of a match head applied directly to your threads.