A waterproof-breathable membrane is not a button or a zipper. It is a functional accessory material: a film laminated to a shell (and often a scrim) so rain stays out and sweat vapour can leave. Put it on the BOM as a named laminate — chemistry, build, hydrostatic head, MVTR — so the rest of the trim pack (seam tape, water-resistant zip, cuff hardware) can follow.
PTFE, PU and TPU belong on a full trim-house list. TrimGem University teaches them as accessory knowledge for a growing Guangzhou platform. The one-stop catalogue will keep adding technical films; 2L / 2.5L / 3L remains a construction choice.
Two ways vapour moves
Microporous films (stretched PTFE is the textbook, GORE-TEX-type architectures) use pores smaller than a raindrop and larger than a vapour molecule. Hydrophilic films (PU, copolyester / PEEA-type, some TPU) have no useful pores: water vapour dissolves and diffuses along a gradient. Hydrophilic films are often less sensitive to pore-clogging dirt and, in some cold-weather talk, to ice in the pores.
TPU appears both as the membrane and as the hot-melt adhesive that laminates film to fabric. PU coatings (wet or dry) can form a continuous layer without a separate film. Hybrid stacks (PTFE plus PU) exist.
Brand names in outdoor (GORE-TEX, eVent, Sympatex, and PU-based house films) are commercial systems. The buyer still needs hydrostatic head and MVTR on a named method — ISO 811 / GB/T 4744 for water resistance, GB/T 12704 or ASTM E96 for vapour, plus tear and abrasion on the laminate. Typical outdoor talk starts at many thousands of mm head and many thousands of g/m²·24 h MVTR; PU coatings sit lower. Compare only on the same method.
2L, 2.5L, 3L
| Build | Stack | Character |
|---|---|---|
| 2L | face + membrane | light; membrane needs a separate lining |
| 2.5L | face + membrane + printed inner | ultralight, inner print protects a little |
| 3L | face + membrane + backer | durable, stiffer, no loose lining against the film |
Seam tape, zippers and snaps must match the waterproof story (welded or taped seams, water-resistant zips). Those complete the accessory pack around the laminate.
PFAS and end of life
Legacy PTFE chemistry sits in the PFAS debate. Outdoor is moving toward non-fluorinated films (including some PE and high-end TPU stories). Multi-layer laminates are hard to recycle; mono-material experiments are design projects, not default mill SKUs.
Hydrostatic head and MVTR numbers only compare on the same method. A 20,000 mm story on one lab form and a 10,000 g MVTR on another is not a ranking. Face fabric (often polyester 50–150 denier in outdoor talk) still has to take the abrasion and tear; the membrane cannot save a weak face.
Zone mapping — tougher 3L on shoulders, lighter 2.5L under arms — is a garment design choice. Trims (pit-zip, cuff hardware, seam tape) have to follow that map.
TrimGem samples PTFE, PU and TPU membranes as accessory materials on a full trim-house BOM: named chemistry, named lamination, named hydrostatic and MVTR methods, plus the tapes and closures that finish the shell.