Wood glue on Kevlar shock chord (LPR/MPR)?

BigMacDaddy

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Has anyone coated their Kevlar with wood glue?

I was looking at / thinking about baffles and it left me wondering about the Kevlar line below the baffle (assuming that Kevlar is attached to motor mount). Which left me wondering about how to protect Kevlar near engine ejection charge.

I often put some wood glue on the knots in Kevlar to keep them from loosening. I have read that putting CA glue on Kevlar weakens it. However, it seems that wood glue is pretty flexible and I am skeptical that it would weaken Kevlar too much. I also think that the reason that the traditional paper shock chord mounts work is because they are covered in glue which is actually what provides the fire resistance. I have found this with duct/duck-tape shock chord mounts -- if I coat the bottom edge with wood glue they hold up much better.

So wondering would a coating of wood glue help protect the first several inches of Kevlar? Added benefit would be that the first several inches of Kevlar would be less flexible and would not fold down accidentally getting stuck across motor ejection port under wadding.
 

lakeroadster

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FWIW: Heat shrink tubing is a cheap and quick remedy.

As to using glue on Kevlar... @Micromeister commented on that, 02-02-2012..

have to Join Jeff in saying
Proper knot choice and knot tying require nothing else to stay where you put them. Any Kit manufacturer who makes such a statement is only looking to sell more of their kits. IT IS NOT EVER OK to glue any line.

It is not advisable to use Glue or epoxy on any knot as it will causes the knotted line, not matter what the material, to fail AT the edge of the glue or epoxy. Adding glue to knots is like adding shards of glass or razor blades to the material of the line. As the line flex back and forth the dried adhesive acts like the edge of a knife..weaking the fibres until they fail. Add just a little strain..like an ejection charge throwing a nosecone to the end of the line or a chute opening... and SNAP!

Can you embed kevlar in glue or epoxy? Sure; many including myself do. We do so with the understanding that the kevlar, or whatever material will eventually fail at that point unless it is somehow "Strain releaved" so no pressure is put on the embedded section of the line. I do this mostly by running my shocklines to the outside edge of the airframe/centering ring joint, keeping the line as far away from the heat as possible.

Also keep in mind KEVLAR does not burn but it does decompose a bit every time it is subjected to temperatures of 482°C/900°F and above. Keeping you kevlar lines as far as practical away from our ejection charges vastly increases the over all life of the shockcord.
 

Bruiser

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I attach my shock cords to a eye bolt installed in the upper baffle plate. That way it's out of the "more concentrated heat" area and it canbe changed out if ever needed. I also use wood glue to help secure the knots when I tie kevlar

-Bob
 

BigMacDaddy

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Heat shrink tubing is a good idea... Thanks!

I can see that CA glue and epoxy would create the cutting effect you and others have described when applied to Kevlar. However, I wonder if the same thing happens with a glue like wood glue that stays pretty flexible even when dry. My searches on glue and kevlar mainly bring up past threads here so I guess other industries / hobbies do not deal with this issue as much.

Ultimately, I usually only launch my rockets a couple of times before moving on to the next build so it may not as much of an issue for me as it is for folks who launch the same rocket over and over.
 

lakeroadster

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Heat shrink tubing is a good idea... Thanks!

I can see that CA glue and epoxy would create the cutting effect you and others have described when applied to Kevlar. However, I wonder if the same thing happens with a glue like wood glue that stays pretty flexible even when dry. My searches on glue and kevlar mainly bring up past threads here so I guess other industries / hobbies do not deal with this issue as much.

Ultimately, I usually only launch my rockets a couple of times before moving on to the next build so it may not as much of an issue for me as it is for folks who launch the same rocket over and over.

Elmers white glue stays kind of flexible. But Titebond and Gorilla Wood Glues, in my experience, do not. These are made for joining wood and if they don't mimic the wood they are joining, then that's an issue.

When I built the ring fin for Ahpla I initially made the mistake of using wood glue to fill some imperfections. When I tried sanding the wood glue it was really hard and the edges were sharp. That's not at all what you want the Kevlar to encounter.

I ended up filling the imperfections with Carpenters Wood Filler.

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mh9162013

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I can see that CA glue and epoxy would create the cutting effect you and others have described when applied to Kevlar. However, I wonder if the same thing happens with a glue like wood glue that stays pretty flexible even when dry.
My understanding is that the issue with using CA glue on Kevlar is two-fold.

First, CA used on Kevlar line is going to be thin and wick its way up along the Kevlar line. This means CA will be in parts of your shock cord you never intended.

Second, dried CA is stiff. Ever notice how easy it is to sand dried CA? Because of this, it can concentrate/focus certain stresses on the Kevlar line to the same area. For example, when the Kevlar line bends, the bend will form at the border between CA-free Kevlar and Kevlar that has been exposed to CA glue. Also, tiny bits of dried CA can work like an abrasive and slowly wear away the fine Kevlar fibers.

So wood/white glue isn't as bad to use on Kevlar line (such as a knot) b/c it's so thick and won't wick into the Kevlar like thin CA will. And depending on the glue, it could have some flex to it which can minimize the stresses placed on the Kevlar line.
 
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