Most WPB raingear on the market relies on micro-porous or non-porous polyurethane coatings on the inner surface of the fabric to provide waterproofing, and extremely fragile and short-lived fluorocarbon “finishes” on the outer surfaces to bead up water and keep the fabric pores open so that water vapor can steam out of the garment.
Because the fluorocarbon DWRs, upon which the breathability of these fabrics rely, are so fragile and short-lived, many have switched to garments that do not rely on DWRs. As far as I can tell, these fall into two broad classes:
- Breathable but very fragile fabrics like Propore (e.g. DriDucks) and Gore-Tex Active/Shakedry that have permanently hydrophobic surfaces. These are a good solution for on-trail use but fail quickly with any rock/tree contact.
- Non-breathable and durable fabrics like silicone-coated nylon, or silnylon. These work well in well-ventilated poncho form, but are too steamy for most to wear as jackets/pants.
The dominant fabrics used for waterproof rainflies in the big brand market are coated with silicone on the outer surface and polyurethane on the inner surface. The outer layer of silicone durably beads water and the inner polyurethane layer provides durable waterproofing.
Why haven’t we seen WPB fabrics with silicone coated outer surfaces and permeable polyurethane inner membranes? This would seem to be a good solution to the WPB dilemma.
I used to think this was because silicone and polyurethane don’t “mix,” but the silicone outer/PU inner fly fabrics seem to bely this. Silnylon is not air permeable, typically, but we know from Richard Nisley’s testing that many silnylons have very low hydrostatic head but still bead water. Couldn’t an Epic-like silicone coating process, applied only to the outer face, co-exist with an inner polyurethane layer that allows moisture transport?
