silnylon with mixed silicone/PU coating on both sides
Fair enough. Interesting idea. I suspect that adding TPU (rather than PU) to the coating might significantly raise the HH. Silicone by itself is a bit porous.
But there must be some benefit to 100% silicone coating; RBTR’s new Mountain Silnylon with very high HH and impressive tear strength is touted as being pure silicone-coated,
A good Q. I imagine that a pure Si coating might be a bit more water-repellent, so that your tarp or tent can shake the water off more easily. Wet tents are a pain. Also, adding TPU might make the coating less flexible down at the micro level, which reduces the tear strength. There’s a lot to be said for a good silicone coating.
All of that said, it is not hard to get those specs. You start with a slightly lighter nylon 6.6 fabric with an absolutely minimal ripstop (you don’t need any ripstop grid really), and then bring it up to the same finished weight with a slightly thicker silicone coating. Apply the coating at high pressure so it fully soaks the threads, and that’s it. 3,000 mm HH is just fine by me, even for kneeling on.
Cheers