Clear PU, White Pigment, Pigmented or Microporous Film: Choosing a Coating for Woven Polyester and Nylon
A coating turns a woven polyester or nylon cloth into something it was not woven to be: water resistant, wind tight, opaque, or stable enough to cut without fraying. It is also a common source of disagreement between a buyer and a mill, because "coated" describes almost nothing. A clear polyurethane film at a few grams per square metre and a pigmented white layer at thirty behave like different fabrics and fail in different ways. This guide covers the four coating families on synthetic wovens, what add-on controls, why coatings delaminate, the waterproof-breathable trade-off, and how to write coating so bulk matches the approved sample.
What a Coating Is, and What It Is Not
A coating is a polymer layer applied to the fabric surface, usually on the back, then dried and crosslinked in a stenter. Once cured it is part of the cloth, not a layer that peels away by hand. It is applied as a liquid or paste by knife, roller or rotary screen.
Three things get confused with it. A durable water repellent finish makes water bead and run off without closing the fabric, so it creates no barrier alone. A laminated membrane is a separate film bonded to the cloth, thicker and usually more expensive, behaving differently in performance and in failure. A hand-feel finish changes touch and appearance without aiming at a barrier. The routes are covered separately in our guide to choosing a functional finish and our article on hand-feel routes for woven polyester and nylon.
The Four Coating Families
Almost every coated synthetic woven falls into one of four groups, separated by polymer, purpose and how much goes on.
| Type | What it delivers | Watch for |
|---|---|---|
| Clear PU | Invisible barrier, wind tightness, down-proof behaviour, cut-edge control | Yellowing or stiffening if the polymer grade does not suit the end use |
| White pigment (PA) | Opacity, coverage of dark base cloth, light-fast backing for prints | Chalking, mark-off, and a noticeable change in handle |
| Pigmented PU | Colour plus barrier in one pass, metallic or tinted backs | Shade control across lots, and colour transfer in packing |
| Microporous film / TPU | Higher water resistance with vapour transmission retained | Pore contamination and delamination under repeated flexing |
Clear polyurethane is the default when the requirement is functional rather than visual. White pigment coatings are usually polyacrylic, making a cloth opaque or giving a clean backing under a print. Pigmented polyurethane adds colour and barrier in one pass. Microporous films are the step up when the specification asks for water resistance and vapour transmission together.
Add-On: The Number Behind Most Disputes
Add-on is the weight of coating left on the fabric after curing, stated in grams per square metre. It decides cost, handle and performance at the same time, and it is the line most often left off the order sheet. Without it, mill and buyer can both be right about the coating type and still deliver different fabric.
The relationship is not linear. Light to medium buys a large jump in water resistance and a small change in handle; medium to heavy buys a small jump in water resistance and a large change in handle, weight and drape. Two further points decide how the figures read: whether the number is wet add-on before drying or dry add-on after curing, and whether it applies to one side or both. Coating both faces roughly doubles the effective add-on.
Weight interacts directly, since coating adds to the finished grams per square metre rather than replacing anything. Our article on specifying fabric weight and matching it to end use covers the base cloth; the coating figure must be stated on top of it, or delivered weight will read out of tolerance against a grey-cloth expectation.
Adhesion: Why Coatings Come Off
A coating fails in three ways, and only one is the coating's fault. Delamination, the layer lifting from the cloth, comes from poor surface preparation or under-cure. Cracking under flex comes from a layer too thick or too rigid for the construction. A drop in water resistance after washing or abrasion comes from the film wearing through.
Surface preparation is the part buyers rarely see and should ask about. Synthetic filament yarns carry spinning and weaving lubricants, and a coating applied over residue bonds to the oil rather than the polymer. Wetting and scouring before coating, plus the thermal history the cloth picks up during dyeing, reduction clearing and heat setting, set the ceiling on adhesion. Cure matters equally: an under-cured film feels dry and passes a touch test, then fails the first wash.
Adhesion is expressed as peel or bond strength in newtons over a stated width, before and after washing or abrasion cycles. Which value is acceptable depends on the garment, so it should be set against the product rather than copied from another order. The framework in our guide to reading a fabric test report applies here: name the standard, the cycle count and the pass level, or the result is not comparable between labs.
The Waterproof-Breathable Trade-Off
Water resistance and vapour transmission pull against each other, and the mechanism decides where the compromise lands. A solid film blocks liquid water and vapour alike; what vapour does move through it diffuses by adsorbing on the inside face, passing through the polymer and desorbing outside. A microporous film has pores small enough to stop droplets but large enough to pass vapour, giving higher transmission until the pores are contaminated by detergent residue, body oil or dust.
In practice, a specification naming only a water column figure invites the cheapest solution that reaches it, usually a heavier solid film with modest transmission. Naming both water column and transmission forces the choice to be made deliberately. Either is valid for the right garment: a packable shell worn in intermittent rain and a work shell worn a full shift in steady rain do not want the same balance. The tolerance band matters as much as the target — how much loss after twenty wash cycles is acceptable should be written down, because it is the number the mill cannot guess.
What Coating Does to Everything Else
Coating is never neutral. It adds weight, stiffens the drape, changes the sound of the cloth, and usually darkens the shade slightly because the film alters how light reflects off the surface. It can also stabilise a loose construction: an open weave that would fray at the cut edge becomes workable, and a down-proof requirement becomes reachable. The variables in our guide to nylon ripstop specification show how base cloth and finish interact on outdoor programs.
Three consequences reach the garment factory. Seams built for uncoated cloth may not seal the same way once a film is present. Shade approval must happen after coating, or bulk will read darker than the approved swatch. And breathability complaints are often construction problems: a lined garment with a coated shell and no venting feels clammy whatever the film can transmit.
Polyester and Nylon Take Coating Differently
Both polymers accept a coating, but differently. Polyester is dimensionally stable after heat setting and gives a predictable coating width and add-on. Nylon has a lower softening range, which restricts the cure window, and is more prone to relaxation movement afterwards, so a coated nylon program should have the finished cloth tested rather than the base. Where heavy add-on is needed on a light base, the choice is worth revisiting; our comparison of polyester versus nylon wovens sets out the wider trade-offs, and our article on woven materials for outdoor use shows where each normally lands.
Writing Coating Into the Specification
Six lines turn a coating requirement into something a mill can quote and a laboratory can verify.
- Name the polymer and the type — clear polyurethane, white pigment, pigmented, or film.
- State dry add-on in grams per square metre, with a tolerance, and one side or both.
- Give the performance target and standard: water column, transmission, and the test method for each.
- State the durability condition — the property required after a named number of wash or abrasion cycles.
- Give a bond or peel strength minimum, with the standard and whether it applies after washing.
- Approve shade after coating, on the coated cloth, for bulk standard and reference swatch.
Anything not written will be produced the mill's way, normally the lightest coating that passes a water column test once. Coating is applied at long-term partner finishing houses rather than in-house; achievable add-on, bond strength and transmission figures depend on the base cloth and the order, and are confirmed on request.
Frequently Asked Questions
Is a coating the same as a water repellent finish?
No. A repellent finish makes water bead and run off but leaves the structure open, so it creates no barrier. A coating lays a continuous polymer layer that closes the structure and resists penetration under pressure. Many garments use both: coating for the barrier, repellent on the face so the surface does not wet out.
What add-on should I specify?
It follows the function. Light add-ons give wind resistance, down-proof behaviour and cut-edge control with little change in handle. Medium is the usual choice for general water resistance. Heavy buys opacity and a higher water column at a clear cost in weight, drape and stiffness. State dry add-on with a tolerance, and say one or two sides.
Why does a coating lose performance after washing?
Three mechanisms, with different fixes. The film is abraded or flexed until it cracks — an add-on or polymer grade problem. It lifts from the cloth — a preparation or cure problem. Or the pores of a microporous film are blocked by detergent residue — a care problem. Wash testing before bulk approval separates them.
Can a coated fabric still be breathable?
Yes, within limits. Solid films transmit vapour slowly by diffusion; microporous films transmit more until pores are contaminated. Specify both water column and transmission, plus what must survive washing, rather than asking maximum on both and leaving the mill to choose the compromise.
Next Step
Decide the function first — barrier, opacity, wind tightness or cut-edge control — then set the polymer and the add-on around it. Send the base cloth and the performance target and we will come back with the coating type and add-on we can hold, the bond and wash-durability figures for that construction, and how to state it on the order sheet. If you already have a coating clause, send the specification as written and we will confirm whether it is measurable before you commit bulk.
