TL;DR

If you want the short answer: an ePTFE membrane gives you HEPA-grade efficiency because its pores are small and its surface is smooth, and it costs you pressure drop in direct proportion to how small you make those pores. There is no membrane that is both the most efficient and the most open in a given thickness. The engineering decision is where on that curve your filter can afford to sit, and the purchasing decision is whether the supplier will state both numbers on the same test basis.

This article comes from the membrane maker’s side of that trade-off. I cannot tell you what class your housing needs, but I can tell you what our grades do, how the numbers relate, and which questions separate a specification from a brochure.

What Sets the Efficiency of an ePTFE Membrane?

Efficiency is set by the pore structure, and ePTFE lets that structure be built rather than inherited. Expanded PTFE forms a node-and-fibril architecture, and the size of the nodes, the length of the fibrils and the open area between them are controlled in processing, which is why pore size can be tuned rather than merely measured.

That tuning is the reason a membrane maker can offer several grades from one resin. Tighten the structure and you capture finer particles at the cost of resistance; open it up and the resistance falls while the penetration of the finest fraction rises. Our published range reflects exactly that: three grades and one adjustable window between them.

Two other properties are often left out of the datasheet: ePTFE is chemically inert, which matters where the air carries acids or solvents, and it is hydrophobic, which matters where condensation would wet out a hydrophilic medium and collapse its efficiency.

Why Pressure Drop Is Not a Separate Question

Pressure drop and efficiency come from the same geometry, so treating them as independent specifications is the most common buying error in this category. At the low velocities used in HEPA filters the flow through the membrane is viscous and laminar, so pressure drop rises roughly in inverse proportion to permeability and in direct proportion to thickness.

That is what makes the permeability figure so useful: it is the variable that predicts what the filter will cost to run. A membrane that is 20% less permeable than its alternative will cost about 25% more pressure drop at the same face velocity and the same thickness.

The practical consequence is that the conversation should be about a target pair, not a target efficiency. Ask for the efficiency at a stated challenge and the resistance at a stated velocity, and treat a supplier who offers only one of them as a supplier who has not tested the other.

How Do the Three Published Grades Compare?

Our three air-filter grades cover a deliberate spread from tight to open, and the numbers below are the published values for each.

The ratio in the last column is the useful part: D42B is the tightest grade, and at the midpoint of its permeability band it presents about 2.7 times the resistance of H12B at the same face velocity, with the thickness difference adding to that. Because that penalty is paid continuously by the fan rather than once by the purchaser, the tightest grade is only the right choice when the efficiency target genuinely requires it.

D43B sits at the open end: the most permeable grade, the lowest published efficiency, and the lowest resistance. Between the three there is a workable window for most HEPA builds, and the choice is usually settled by the particle size the application must capture rather than by the headline percentage.

What Does Efficiency Actually Mean Without a Test Basis?

An efficiency figure is a ratio produced by a method, and two methods can give very different numbers for the same membrane. The European HEPA convention classifies filters by their penetration at the most penetrating particle size, typically around 0.1 to 0.2 µm, while the older American shorthand of 99.97% at 0.3 µm refers to a different particle size and a different test arrangement.

ISO 29463-1:2024 is the ISO counterpart that establishes the classification of high-efficiency filters and filter media by performance, determined in accordance with its companion test parts, and it also sets out the general requirements for assessing, marking and documenting the results. The point is the word classification: a class is assigned after a defined test, and a percentage on a datasheet is not a class.

This is where our own table should be read carefully. The published figures of >99.5%, >99.7% and >99.9% are efficiency values for the media, and the accompanying page notes that the membrane can reach European classes H11, H12 and H13 depending on the construction. Those two statements are not the same claim, and a buyer should ask which test method, which challenge aerosol and which face velocity produced the percentage before mapping it to a filter class.

There is also a structural reason why flat-sheet efficiency cannot be read straight across to a finished filter. A HEPA filter is scan-tested for leaks at the frame, the pleat pack and the sealant, and a membrane can be excellent while the filter fails that test. Media efficiency is an input to the classification, not a substitute for it.

What Does a Pascal Cost You?

Pressure drop becomes money at the fan, and the arithmetic is simple enough to do in a meeting. Fan shaft power equals airflow multiplied by pressure drop, so every 10 Pa of media resistance at 1 m³/s draws about 10 W, which is roughly 88 kWh over a year of continuous running.

Add fan efficiency and the number at the meter is higher: at 60% efficiency the same 10 Pa costs about 145 kWh a year. That is what turns a 2.7 times resistance ratio into an operating cost, and why a slightly thicker but more open membrane can beat a thin tight one over a filter’s life.

The number also sets the priority for testing. If a supplier can show a permeability figure measured to a recognised method, the pressure drop at your face velocity can be estimated before you cut a sample. ISO 9237:1995 describes the method for measuring the permeability of fabrics to air and is applicable to industrial fabrics, nonwovens and made-up articles, which is the basis on which a permeability figure in L/m²·s should be quoted.

Which Membrane: Surface or Depth Filtration?

ePTFE works by surface filtration, and that is the difference that shows up after the first cleaning cycle rather than on the first day. Because the membrane captures particles on its face rather than inside its structure, the dust cake forms where it can be removed, and the pressure drop recovers after cleaning instead of creeping upward.

A depth medium such as meltblown or glass fibre holds particles within its thickness. It is often cheaper and perfectly adequate for a single-use filter, but its resistance rises as the pores fill and it cannot be cleaned back to its original value. On a pulse-jet baghouse or a washable cartridge, that decides the service life.

Our range is built for lamination rather than for stand-alone use: the membrane is bonded to PP felt, polyester, PPS, Nomex needle felt or glass fibre needle felt, and the substrate carries the mechanical load while the membrane does the filtration. A membrane datasheet that does not name the substrate has left out half of the finished media.

What Should Be on the Purchase Specification?

Seven items belong on the specification, and each one is a variable that changes the delivered product.

1. Air permeability with its test basis, because it is the number that predicts resistance.
2. Thickness with a tolerance, since resistance scales with it.
3. Efficiency with the challenge aerosol, particle size and face velocity stated.
4. Bubble point or pore size, as the structural check that ties the two together.
5. Width and roll format, with our 2600 mm and 2600–3500 mm options in mind.
6. Substrate and lamination method, including the adhesive if one is used.
7. Lot documentation, so a roll can be tied to the test data that qualified it.

Item three is the one that gets compressed in a quotation and the one that causes disputes later, because a percentage without the method is an opinion with a decimal point.

How Do You Qualify a Membrane Supplier?

Qualify on samples, lot data and a defined acceptance test, in that order. A sample proves the material exists; lot data proves the process is in control; an acceptance test proves the two of you mean the same thing by a passing roll.

Start with a sample and test it on your own bench, because a membrane that measures well on the supplier’s rig and poorly on yours is a calibration conversation rather than a material one. We ship samples through the contact page, and the useful request includes the face velocity you will run at and the substrate you intend to laminate to.

Then agree a sampling plan for incoming lots rather than testing the first roll. ISO 2859-1:2026 defines AQL-indexed sampling schemes for lot-by-lot inspection, and permeability, thickness and width are the three attributes worth sampling on every delivery, with efficiency tested per lot or per agreed frequency.

Where ePTFE Is Not the Right Answer

It is not the right answer where the filter is single-use and the efficiency target is modest, where the air is heavily loaded with coarse dust that would blind a surface medium before its efficiency mattered, or where the budget cannot carry the lamination step.

A coarse pre-filter ahead of a HEPA stage is usually the better place to spend money than a tighter membrane, because it removes the load that would otherwise sit on the surface and drive the pressure drop up. And where the whole filter is discarded after one campaign, a depth medium at a lower resistance often delivers the same captured mass for less fan energy over that short life.

Frequently Asked Questions