Leash repair, melt / weld

[sflinux]

I came across a couple videos of where guys heat up urethane and then butt it together, and once cooled forms a welded repair. 

I was skeptical to say the least. 
I used a heat gun to heat up both ends while being held (separated) by a vise.  The urethane looses its glossy finish and turns hazy.  As it gets hotter it starts bubbling.  One challenge is it getting the axis of each end to align properly.  i used a vise slightly open to cradle the ends and I butted them together as they cooled.  You end up getting a mushroom at the union.  I used a dremel tool to remove the mushroom head.  My first attempt got too hot and was off-center so I just cut each side out with a hacksaw and repeated.
It takes less heat than you would expect.  I am of the opinion that once the urethane bubbles, that is too hot.  At the bubbling temperature, you can get soot which will diminish the integrity of the bond.  It seems like once the urethane looses its glossy finish and turns hazy, that is the right temperature.
Today I took the leash out with a 10' prone longboard in 2-3+ conditions and got close to 50 pulls with it and it is still going strong.  I counted each wave tug, expecting a failure, but this type of repair seems like a PLAUSIBLE option.  This leash is a creatures of leisure reliance outer reef  big wave leash with a 9 mm cord.  It had a snap at the halfway point which was a clean cut.  I was already planning on replacing the leash as it served me well for 5 years and the velcro is worn out.  Doubt I will test this leash with a SUP or in big waves again, but it may have a second life with surfboards.  Limited data on longevity, but mine and my buddies independent repair lasted an entire session.  Could be surf hack for trips while traveling.


For more traditional leash repairs you can refer to this video:
https://youtu.be/7UuRMn4nJ_4?si=iOsiCr-ksApDERKk

And this thread has good leash tips as well:
https://www.standupzone.com/forum/index.php/topic,18684.0.html

[jondrums]

It is very common in industrial settings to butt-weld polyurethane cordstock into continuous loops for conveyor drive belts.   And you hit on the two main difficulties: aligning the ends, and avoiding overheating.    Some sophisticated tools have been developed to solve this, but lots of people have totally fine success with a heat gun like you did.     I couldn't find any data for residual strength of the weld versus virgin material, but I suspect that a correctly done weld should retain almost all or all of the original strength properties because the material used for leashes is technically a Thermoplastic Poly Urethane (TPU).  Thermoplastic materials become pliable/moldable at elevated temperatures and crosslink when cooled in a reversible and repeatable way so if the weld is done at the correct temperature it will be fully crosslinked.

Take a look at these links if you're interested.
https://en.wikipedia.org/wiki/Thermoplastic_polyurethane
https://www.fennerdrives.com/welding-kits-and-components/
https://www.behabelt.com/en/twin-v-belts/