How Hydrostatic Pressure and Water Repellency Differ in Nonwoven Fabrics

Direct answer: Water repellency is a surface property — it measures how strongly a fabric resists being wetted, and it is reported as a spray rating from 0 to 100 (AATCC TM22 / ISO 4920). Hydrostatic pressure is a structure property — it measures how much water pressure the fabric withstands before liquid passes right through it, reported in mmH₂O or mbar (ISO 811 / AATCC TM127). They answer two different questions, and neither predicts the other. A spunbond nonwoven can score a perfect 100 on the spray test and still leak at 20 cm of water head, because repellency lives at the fibre surface while hydrostatic resistance is decided by the size of the pores between fibres.

What is the difference between water repellency and hydrostatic pressure?

The two terms get used interchangeably in sales conversations, which is exactly why specifications go wrong. Water repellency asks: when water lands on this surface, does it bead up or spread out? That is a question about surface energy. Hydrostatic pressure asks: when water is pushed against this fabric hard enough, at what point does it come through the other side? That is a question about pore geometry — the size, tortuosity and connectivity of the gaps between filaments.

The physical reason they diverge is capillary pressure. Water enters a pore only when the applied pressure exceeds the Laplace threshold, roughly Δp = 4γ cos θ / D, where γ is the surface tension of water, θ the contact angle on the fibre, and D the pore diameter. Hold the chemistry constant and the decisive variable becomes D: halve the pore diameter and you roughly double the pressure needed to push water through. This is why a repellent finish alone cannot turn a porous spunbond into a barrier, and why a fine-fibre meltblown layer can, even with modest chemistry.

In commercial terms, the two measurements also serve different stages of a decision. Repellency answers “will the bag, the cover or the garment look wet and stain?” Hydrostatic pressure answers “will the contents actually get soaked?” A buyer who asks only the first question will eventually ship a product that fails the second.

What does nonwoven water repellency actually measure?

Repellency is measured by spraying a fixed volume of water onto a tensioned specimen from a fixed height and grading the wetting pattern that remains. ISO 4920 specifies this spray test for determining the resistance of any fabric to surface wetting, whether or not it carries a water-repellent finish. AATCC TM22 is the US counterpart and grades the result on a 0 / 50 / 70 / 80 / 90 / 100 scale: 100 means the surface shows no clinging or wetting at all, while lower grades record progressively larger wet areas and, eventually, complete wetting of the specimen.

Two things are worth knowing about this test. First, it is a visual, pattern-based rating, not a pressure measurement, so it is deliberately coarse — it separates a good finish from a poor one far better than it separates two good finishes. Second, a spray rating says nothing about durability under mechanical action. For that, the rain-shower approach is used — the Bundesmann method under ISO 9865 applies an artificial rainstorm with rubbing action and then reports water absorption, penetration and runoff separately.

For nonwovens there is an extra wrinkle: polypropylene is inherently hydrophobic, so an untreated PP spunbond will usually bead water reasonably well on first contact. That baseline can mislead a buyer into believing the fabric is protected, when in fact the same fabric is a wide-open pore network the moment any pressure is applied. Contact-angle measurement, rather than spray rating, is the more sensitive tool when you need to compare two candidate finishes rather than simply pass or fail one.

What does the hydrostatic pressure test measure?

The hydrostatic test applies water pressure to one face of a specimen and raises it steadily until liquid penetrates. Under ISO 811, the specimen is subjected to a steadily increasing pressure of water on one face under standard conditions, and the test ends when penetration occurs in three places — a rule that prevents a single pinhole defect from deciding the result. AATCC TM127 is the US equivalent and is widely cited in protective-clothing specifications.

The output is a pressure, and the units are where most confusion starts. The same result can legitimately be written five ways:

• mmH₂O (or cmH₂O) — the traditional head-of-water expression
• mbar — equal to hPa, common in European test reports
• Pa / kPa — the SI expression used in engineering documents

The conversion is simple and worth memorising: 1 mbar = 1 hPa ≈ 10.2 mmH₂O, and 1 mmH₂O = 9.81 Pa. So a fabric quoted at 5,000 mmH₂O is holding back roughly 49 kPa, or about 490 mbar. A supplier who quotes “5000” without units has told you nothing you can compare.

Hydrostatic pressure vs water repellency: how do the two compare side by side?

Read the table as a division of labour rather than a ranking. A high spray rating with a low hydrostatic head describes a fabric that stays looking dry but is not a barrier — common in untreated spunbond. A high hydrostatic head with a mediocre spray rating describes a barrier fabric whose outer surface wets out — common in membrane-laminated or heavily compacted constructions, where the barrier works but the face fabric looks soaked. Both are perfectly legitimate products; they are simply specified for different jobs.

Why do nonwovens behave differently from woven fabrics?

A woven fabric is built from yarns interlaced at right angles, so its pores are the regular gaps between yarns and water can also wick along the yarn structure. A spunbond nonwoven is a random network of continuous filaments bonded at discrete points, which gives it three distinctive behaviours.

First, pore size is distributed rather than uniform. A nonwoven contains a spread of pore diameters, and hydrostatic failure is triggered by the largest connected path, not the average pore. That is why two nonwovens with identical GSM and tensile strength can differ by a factor of several on hydrostatic head.

Second, there is no continuous yarn to wick along, which generally makes a nonwoven less prone to the wicking-driven leakage that afflicts tight woven constructions — but it also means there is no compact, low-porosity surface to start from. A spunbond relies on fibre fineness and bonding density for whatever barrier it has.

Third, bonding pattern matters as much as weight. Calender bonding compresses the web at the bond points, locally collapsing porosity while leaving the unbonded lanes open. A heavier fabric with a sparse bond pattern can therefore test worse than a lighter fabric with a dense, well-designed pattern. This is the single most common reason a buyer’s “we increased the GSM and it still leaks” experiment fails.

Which nonwoven structures deliver which level of water resistance?

The design logic follows directly: strength comes from spunbond, barrier comes from fine fibres or a continuous layer. If an application genuinely needs water not to pass, the answer is a meltblown core, a membrane, or a film — not a heavier spunbond with a stronger finish. Understanding that distinction early saves both money and a failed validation run.

How do you specify water resistance for a nonwoven order?

Specify both properties separately, and specify them in a way a laboratory can reproduce. Six items do most of the work:

1. Name the method, not just the property. “Water repellent” is not a specification; “ISO 4920 spray rating ≥ 90” is.
2. State the units. For hydrostatic pressure, write “≥ 1,000 mmH₂O (≈ 98 mbar) per ISO 811” so there is no ambiguity in either direction.
3. State the face tested. Hydrostatic results differ depending on which side of a layered or laminated fabric faces the water.
4. State the conditioning. Standard textile conditioning before testing is not optional; an unconditioned specimen can read several grades off.
5. State any after-care requirement. If the article will be washed, specify the rating required after the stated number of wash cycles, not only on the new material.
6. State the specimen count and the pass rule. One specimen is an anecdote; agree how many are tested and how many may fail.

For sewn or heat-sealed products, add a seventh: seam performance. A fabric that passes at 1,000 mmH₂O flat can leak at the needle line, which is why protective-clothing specifications test seams as separate items rather than assuming the fabric result carries over.

What mistakes make suppliers’ data sheets incomparable?

The most expensive mistake is unit confusion. A report reading “490” is excellent in mbar and unremarkable in mmH₂O — a factor of ten apart. Always convert both offers to the same unit before comparing them.

The second is method substitution. Spray rating and hydrostatic head are routinely presented side by side as if they measured the same thing, so a supplier can look stronger simply by quoting the test it performs best on. Ask which method produced each number.

The third is selective reporting: presenting the best specimen rather than the mean, quoting new-material results for a product that will be washed, or quoting flat-fabric data for a finished sewn article. Ask for the raw specimen results, not only the summary line.

Finally, watch for the word “waterproof” used without a number. In most regulated markets it is a claim that has to be supported by a stated method and threshold. If a data sheet uses the word and no test follows it, treat the claim as marketing rather than engineering.

Frequently asked questions

Is nonwoven fabric waterproof or just water repellent?

Usually just water repellent, and often only mildly so. A plain spunbond is hydrophobic on the surface but porous through the thickness, so it resists wetting without resisting pressure. Nonwovens become genuinely waterproof when a continuous barrier is added — a meltblown core in SMS, a membrane, or a film lamination — and the resulting product is specified by a hydrostatic pressure value, not by the word “waterproof”.

Can a fabric score 100 on the spray test and still leak?

Yes, and this is the most common misunderstanding in the category. A 100 spray rating says the surface does not wet; it says nothing about whether water can be forced through the pores. An untreated or lightly treated spunbond can achieve a high spray rating on first contact and still pass water under a few centimetres of head. Only a hydrostatic test tells you where that threshold is.

Why do suppliers report mmH₂O and mbar — how do I convert?

They are the same physical quantity in different units: 1 mbar equals 1 hPa and is approximately 10.2 mmH₂O, while 1 mmH₂O equals 9.81 Pa. To compare two offers, convert both to mmH₂O or both to mbar. A 5,000 mmH₂O result is roughly 490 mbar or 49 kPa, so a report showing “490” is the same fabric.

Does washing reduce nonwoven water repellency?

It usually does, and the amount depends on the finish chemistry and the fibre, not on the GSM. Repellent finishes are progressively removed or redistributed by surfactant action and mechanical agitation, so the meaningful specification is the rating required after the intended number of wash cycles. Test the laundered sample, not only the new one.

Which test belongs in a purchase contract?

Both, if both properties matter to the end use — a spray rating for appearance and surface behaviour, and a hydrostatic value for barrier performance, each with its method, units, face and conditioning stated. If only one is contractually relevant, contract that one and state explicitly that the other is not guaranteed, rather than leaving the buyer to assume it.

Does a higher GSM always mean better water resistance?

No. GSM raises the number of fibres but does not by itself change the largest pore, which is what governs hydrostatic failure. Fibre fineness, bonding pattern and added layers control barrier performance far more strongly than weight does. Increasing GSM is a reasonable way to raise strength and opacity; it is an unreliable way to fix a leaking specification.

Key takeaways

• Water repellency and hydrostatic pressure measure different things: surface wetting versus liquid penetration.
• Repellency is graded 0–100 by spray test (ISO 4920 / AATCC TM22); hydrostatic resistance is a pressure in mmH₂O or mbar (ISO 811 / AATCC TM127).
• 1 mbar ≈ 10.2 mmH₂O — the conversion error that most often invalidates a comparison.
• In nonwovens, barrier performance is governed by pore size, which is why fine-fibre meltblown layers outperform heavier spunbond.
• Specify method, units, face, conditioning and after-care — or the number cannot be reproduced.

Mingyu Group (Zhuhai Mingyu New Materials Co., Ltd.) produces PP spunbond nonwoven for bag lining, furniture and packaging applications, with measured GSM, tensile data and test methods stated on every certificate. If you are specifying a water-resistant grade, send us the end use and we will tell you which structure actually delivers it.