Your rain jacket stopped repelling water. The coating is almost certainly still on it

A rain jacket that is three years old behaves differently from a new one. Water no longer stands up in beads and rolls off; it soaks into the outer fabric, which goes dark and heavy, and after an hour in the rain you are damp inside and reasonably certain the jacket has failed.

Two things about that are worth knowing. The first is that the water-repellent finish is almost certainly still on the fabric — very little of it has actually gone anywhere. The second is that the jacket is probably not leaking at all. Both facts follow from the same thing: putting a coating on cloth is a fundamentally different exercise from putting one on steel or glass, and almost everything surprising about treated fabrics comes from that difference.

A fabric cannot be coated the way a surface is

On a hard surface, a coating is a film. It lies over the substrate as a continuous layer, and the questions are the ones any film raises — thickness, adhesion, whether there are holes in it.

Cloth will not accept that treatment. A continuous film across a woven fabric is no longer a fabric; it is a plastic sheet with threads in it. It does not drape, it does not breathe, and it cracks where it folds. And fabric has an enormous amount of surface to cover: a square metre of woven textile presents a surface area orders of magnitude larger than a square metre of sheet steel, because every one of its billions of individual fibres has a full circumference exposed.

So a textile finish does something else. It coats each fibre, conformally, in a layer often only nanometres thick, and deliberately leaves the spaces between fibres and between yarns wide open. The air gaps are the point. They are what let vapour through, what let the fabric drape, and what let it stay a fabric.

That means a textile finish is not a barrier. It cannot be — it is full of holes by design. It works by changing what the surface of each fibre does when a liquid arrives. Everything else follows from that single design constraint.

What repellency actually is, in one number

Whether a liquid spreads on a solid or pulls itself into a bead is decided by a comparison between two quantities: the surface tension of the liquid and the surface energy of the solid. If the solid's surface energy is comfortably below the liquid's surface tension, the liquid cannot wet it and beads up. If it is above, the liquid spreads.

Now the numbers, because this is where the whole subject becomes clear.

Water has a surface tension of about 72 mN/m. That is very high — water is unusually cohesive — so keeping a surface below it is easy. Silicone finishes, hydrocarbon and paraffin waxes, and various dendrimer chemistries all sit far enough below to make water bead beautifully. Repelling water is, chemically speaking, not a hard problem.

Oils are the problem. Cooking oil, body oil, salad dressing, machine oil: surface tensions around 20 to 30 mN/m. To repel those, a surface must get below that, and there is a very short list of chemistries that do. A surface densely packed with CF₃ groups — the end of a perfluorinated chain — reaches roughly 6 to 10 mN/m, the lowest surface energy achievable in any practical material. Nothing else in commercial use gets close.

This is the entire reason the textile industry has spent two decades in difficulty over one family of chemicals, and it is why the difficulty is real rather than a matter of insufficient effort. Fluorochemical finishes were not chosen because they were cheap or familiar. They were chosen because a physical property that only fluorine delivers was needed to repel oil, and it is the same reason a self-cleaning coating's performance comes down to how water sits on it rather than to anything the coating actively does.

Why the jacket stopped working

With that in place, the failure makes sense.

A durable water repellent finish fails, in ordinary use, for two reasons that are both recoverable and one that is not.

Contamination. The finish works by presenting a low-energy surface. Cover it with something higher-energy — body oils, sweat, sunscreen, road grime, and above all detergent residue, which is engineered to make water wet things — and the fabric's effective surface energy rises. The finish is intact; it is under a film of something that undoes it. This is the most common cause by a wide margin, and it is why washing a technical jacket properly, with a rinse that actually removes surfactant, often restores most of the performance.

Chain reorientation. The molecules of a repellent finish work when their low-energy tails stand away from the fibre. Mechanical abrasion and washing can leave them lying flat, and the surface stops presenting the face it is meant to. Heat re-orders them — which is why every technical clothing manufacturer tells you to tumble-dry or warm-iron a jacket to revive its water repellency, and why that advice sounds like folklore and is in fact solid physical chemistry.

Genuine abrasion loss, mostly at shoulders, cuffs and anywhere a bag strap sits. This one is real wear, and reproofing is the only answer.

The leak that is not a leak

Now the second surprise, which is worth more than it looks if you own such a jacket.

A waterproof breathable jacket has two separate things doing two separate jobs. A membrane or coating on the inside is what actually stops liquid water — that is the waterproof part, and it is remarkably durable. The repellent finish on the outside face fabric does something else entirely: it stops the outer fabric itself from soaking up water.

When the outer fabric soaks up water — the trade calls it wetting out — the membrane goes on stopping rain perfectly well. What stops is breathing. Vapour transport through a membrane is driven by a difference in vapour pressure across it, and a saturated outer fabric kills that gradient. Your own sweat then has nowhere to go, condenses on the inside of the membrane, and you get wet from within while the jacket is, technically, not leaking at all.

This is why the outer finish matters so much more than its modest job description suggests, and why a jacket can be perfectly waterproof and completely useless at the same time.

Almost every complaint that a technical fabric has "failed" describes a surface that is dirty or a finish lying flat. The material is usually still there. Testing for that distinction — before replacing anything — is worth the twenty minutes it takes.

How it is put on, and how it is tested

The finish is applied by padding: the fabric runs through a bath of the finishing chemistry and then between rollers that squeeze it to a controlled wet pick-up, and it is then dried and cured at temperature, typically well above 150 °C, so a crosslinker bonds the finish to the fibre. That cure step is not optional. An uncured or undercured finish washes off in a few cycles, which is the difference between "durable" and "temporary" in the name.

Testing is correspondingly specific, and the numbers are worth being able to read:

• Spray rating (AATCC 22 / ISO 4920) sprays water at a mounted swatch and scores the wetted pattern from 0 to 100. This is the everyday water-repellency number.
• Oil repellency (AATCC 118) places drops of a graded series of hydrocarbons of decreasing surface tension on the fabric and reports the lowest-tension liquid that still beads. This is the test where non-fluorinated finishes score poorly, and it is the honest measure of the substitution problem.
• Hydrostatic head (ISO 811) measures the water pressure a fabric withstands before it passes water — this is waterproofness, a different property from repellency.
• Breathability is reported as moisture vapour transmission rate, or as RET under ISO 11092, where lower is better.
• And crucially, every one of these is quoted after a stated number of laundering cycles. A repellency figure with no wash count attached is not a specification.

The substitution problem, third time around

Long-chain perfluorinated finishes — the C8 chemistry — were restricted internationally because PFOA and related substances are persistent, bioaccumulative and toxic, and PFOA was added to the Stockholm Convention in 2019 after PFOS a decade earlier. Regulatory attention has since widened from individual substances towards the class, with a broad restriction proposal under the EU's REACH framework in process.

The industry's responses have been a shorter-chain fluorochemistry, generally referred to as C6, and a growing family of non-fluorinated finishes based on silicones, hydrocarbon waxes and dendrimers. The honest scorecard, from the numbers above:

For water, the non-fluorinated finishes are genuinely competitive. For most clothing, most of the time, this is a solved substitution.

For oil and for stains, they are not, and the physics says they cannot be. A silicone finish does not get below 20 mN/m, so it cannot repel a liquid whose surface tension is 25. This is why workwear, medical textiles, food-industry clothing and upholstery — the applications where oil and stain repellency is the actual requirement — are the hard cases and remain so.

The pattern will be familiar to anyone who read about chromate in corrosion primers or bisphenol A in can linings. A chemistry is adopted because it is exceptional at a physical job, its persistence or toxicity catches up with it, and the replacement search finds that the exceptional part is exactly the part that is hard to replace. Coatings science has now run this cycle three times in three different industries, and knowing the pattern is a reasonable defence against believing the next marketing claim too readily.

Antimicrobial finishes on cloth are a separate question

A fabric can also be treated to reduce microbial growth, and it is worth separating that from repellency, because the two are sold side by side and tested completely differently.

The chemistries are those used on hard surfaces — silver, and quaternary ammonium organosilanes that bond to the fibre — but the substrate changes the engineering. Fibre is absorbent, so the finish must bond into and around it rather than sit on it, which is exactly why suppliers list water-based and solvent-based variants for absorbent surfaces separately: penetration and carrier behaviour differ, and leather in particular, being a protein matrix that must stay flexible and breathable, is its own case again.

And the durability question is not abrasion but laundering. Efficacy is measured by ISO 20743 or AATCC 100, and the specification that matters is the log reduction retained after a stated number of wash cycles — twenty, fifty — not the figure on a fresh sample. The relevant point from how antimicrobial surfaces are actually tested applies here with more force, because a fabric gets washed in a way a wall does not.

What to ask before buying treated fabric

1. Water repellency or oil repellency? They are different tests and different chemistries. If stains are the requirement, ask for the AATCC 118 rating, not a spray rating.
2. What is the wash count? Every performance number should carry one. "Durable" without a laundering cycle count means nothing.
3. Fluorinated or not, and if not, what was given up? A non-fluorinated finish is a reasonable and often preferable choice — but it should be chosen knowing that oil repellency is the trade, not sold as if there were none.
4. What is the cure schedule? For anything applied to fabric rather than bought pre-finished, the temperature and dwell time are part of the specification, and skipping them is the most common way a durable finish turns out not to be.
5. Does it change hand, drape or breathability? Ask for RET or MVTR figures if the fabric is meant to be worn.
6. For antimicrobial claims: which standard, which organism, what log reduction, after how many washes? All four, or the claim is decoration.

Concretely on how a range should be organised: Smart Warrior Coatings, part of Reinste Nano Ventures — which, in the interest of disclosure, belongs to the same group as this publication — lists separate water-based and solvent-based products for leather and textiles rather than offering its general-purpose surface coatings for absorbent substrates. That distinction is the correct one, and it is a reasonable thing to require of any supplier: a coating qualified on steel and glass has not thereby been qualified on cloth, and the technical data sheet should say which it is.

Why it matters for students and researchers

Textile finishing is an unusually accessible research area, because the substrate is cheap, the equipment is common in Indian technical institutions, and the open problems are commercially urgent.

The largest of them is the oil-repellency gap. Getting a non-fluorinated surface below 20 mN/m is a genuine materials challenge with a large prize attached, and the promising directions — engineered surface texture that amplifies whatever intrinsic repellency the chemistry has, hybrid and organic-inorganic finishes, silicone architectures with unusual chain packing — are all approachable at laboratory scale. Alongside it sits durability, since a finish that performs superbly for five washes has solved nothing, and the honest reporting of wash-cycle data is itself an area where the literature could be better than it is.

There are two further problems worth naming. Microplastic and finish shedding during laundering is now a measurement discipline in its own right and one where methods are still settling. And application efficiency — padding is a wet process with meaningful water and energy consumption, so foam, spray and plasma-assisted routes that reduce both are practical research with an immediate industrial audience.

The Indian context adds a specific opening. Finishes are developed and tested largely against temperate laundering habits and climates. High humidity, high ambient temperature, hand washing, hard water, strong detergents and long sun exposure are the normal conditions here, and how these finishes behave under them is thinly documented for a country with a textile industry this size.

Frequently asked questions

How do I restore water repellency on a jacket?

Wash it first, and properly — a technical wash formulated without softeners or optical brighteners, or a very thorough rinse with ordinary detergent, because detergent residue is itself a leading cause of the problem. Then apply heat: tumble-dry on low, or iron on a low setting with a cloth in between, following the garment's care label. That reorients the finish and recovers most jackets. If beading still fails after that, the finish has genuinely worn, and a wash-in or spray-on reproofer is the next step.

Does washing a technical jacket ruin it?

The opposite, usually. Not washing is worse, because the accumulated body oils and grime are what suppress repellency in the first place. The care instruction people miss is not "wash less" but "rinse thoroughly and skip the fabric softener", since softeners deposit exactly the kind of hydrophilic film the finish is trying not to have.

Is "PFAS-free" a guarantee of good performance?

It is a guarantee about chemistry, not performance, and for water repellency it is usually fine. For oil and stain repellency it is a real trade-off, because the surface-energy arithmetic does not permit non-fluorinated finishes to reach where fluorinated ones go. A PFAS-free rain jacket is a sensible purchase. PFAS-free workwear for a kitchen or a workshop should be bought with the oil-repellency rating in hand.

Are antimicrobial clothes worth it?

For odour control they can be, since odour is largely bacterial and reducing that population reduces smell. For infection control in a home setting the case is much weaker, and the honest questions are the same ones asked of any antimicrobial surface: which organism, which standard, what log reduction, and — the one specific to fabric — after how many washes. Absent all four, treat the claim as marketing.

Why can't a fabric be both fully waterproof and fully breathable?

Because the two requirements pull in opposite directions, and the industry's solution is a compromise rather than a resolution. Stopping liquid water means blocking pathways; letting vapour out means keeping them open. Membranes exploit the fact that a water vapour molecule is far smaller than a liquid droplet held together by surface tension, so a structure can be built that passes one and blocks the other — but the transport is slow, driven by a vapour pressure difference, and it stops working entirely when the outer fabric wets out. Which is where this article began.