Two suppliers answer the same question and the answers do not look like each other. One quotes a volume in millilitres. The other quotes how much absorbent polymer the pad contains, in grams. The obvious move is to convert one into the other and pick the bigger number, and that move is where the trouble starts, because no conversion exists.
The sanitary pad absorption capacity ml figure is a mass in disguise
A bench does not pour liquid into a graduated cylinder and read off a scale. It weighs the sample dry, lets it take up liquid, weighs it again, and divides the gain by an assumed density for that liquid. The answer is reported as a volume because the buyer asked for a volume. What was measured was mass.
That substitution explains most of the confusion around the unit. Where the liquid is close to water, the assumption costs almost nothing and the figure is honest. Where the liquid is a viscous synthetic urine built to behave like a body fluid, the density is different, the assumption is a guess, and the guess now carries a unit that makes it look like a measurement. A buyer comparing sanitary pad absorption capacity ml across three suppliers may be comparing three assumptions and nothing else.
Inside the gel, volume and mass come apart
Absorbent polymer does not dissolve, it swells. Liquid is drawn into the network and held there, and the swollen particle takes up far more space than the dry one did. Part of the liquid sits inside the gel and part sits in the gaps between particles, and only the first part is held firmly.
This is why the same mass of absorbed liquid can occupy different volumes in two pads. Press the core and the gaps close, so the volume falls while the mass is unchanged. A figure in millilitres therefore describes a state, and the state depends on how much air the core still contains when the liquid arrives.
The line doses mass because it cannot dose volume
A gravimetric feeder meters polymer by weight. Nothing on a sanitary pad production line can meter a millilitre of absorbent, because the material arrives as particles and not as liquid. The setting an operator can change is grams per pad. The setting the market asks about is a volume that does not exist until the product has been used.
The ratio between those two figures is established once, by testing a sample, and then assumed for the rest of the run. That assumption is the weakest link in the chain, because it is fixed when the core is at its most typical and then applied to every pad that follows, including the ones made at the end of a long shift.
A sanitary pad thickness micron target changes the ratio
Compaction is the usual way to make a pad thinner, and compaction removes air. Remove air and the same polymer mass has less room to swell into, so the core accepts less liquid before the surface stops clearing. The millilitre figure falls. The gram figure does not move at all.
Buyers sometimes try to answer this by asking for more polymer in a thinner core. That is not impossible, but it is a different product rather than a stronger version of the same one, and the reason is geometric: the material needs somewhere to expand. A brief asking for a low profile and a high figure at the same time is asking the supplier to choose which one to under-deliver.
Where the sanitary pad manufacturing machine loses its dose
Dosing is not the same as landing. Polymer travels in an air stream and is deposited on a forming drum, and a fraction of it never arrives. Some leaves with the exhaust, some coats the duct, and some separates from the fibre because the two materials have different densities. The mass set at the feeder and the mass in the finished pad are two different numbers.
The gap between them is not constant either. It drifts with humidity, with the condition of the polymer, and with line speed, so the effective loading at the start of a run and at the end can differ even when nobody touched the feeder. A figure taken from one sample is an average of a number that was moving while it was measured.
Comparing two quotes that never share a unit
The practical fix is to normalise before comparing. Ask for the polymer mass per pad and for the volume the core accepted in the test, then divide one by the other. The result is a ratio with no unit at all, and ratios from two suppliers can be set side by side even when their headline figures cannot.
| Figure as quoted | What it actually states | Who controls it |
|---|---|---|
| Volume in millilitres | Mass gain divided by an assumed liquid density | The bench, and the assumption behind it |
| Grams of absorbent per pad | Mass metered at the feeder | The line setting, less whatever never lands |
| Volume per gram | How much liquid each gram of polymer accepted | The design, not any single setting |
Read the third row and the commercial question changes shape. Two pads that accept the same volume with different polymer masses are not the same product, and the one with the higher ratio is getting more out of each gram. That usually points to a loftier core, a better distribution layer, or a different polymer grade, none of which appears in either headline figure.
What belongs in the brief instead of a volume
Most briefs we receive state a sanitary pad absorption capacity ml target and nothing else, which leaves the supplier to guess the basis. A brief that names the polymer mass, the profile and the failure to avoid gives the line something it can set, and gives the buyer something that can be checked on arrival.
- The polymer mass per pad, with the tolerance the feeder can hold across a full run.
- The profile, because thickness and capacity trade against each other.
- The failure that matters more, a wet surface early or liquid returned under pressure.
- The basis line behind any volume figure quoted back to you.
That last item is the one worth insisting on. A volume figure with no basis line cannot be compared with anything, including the same supplier's figure from the previous order, and the argument it produces ends up being about two documents rather than about a product.
Two requests that pull against each other
The two common failure modes do not respond to the same fix. A core built to accept the most liquid is lofty, and a lofty core gives some of that liquid back when weight is applied. A core built to hold liquid tightly is dense, and a dense core clears the surface more slowly. A brief can weight one of these, or it can split the difference, but it cannot maximise both.
We build pads and pants in Guangdong and have done so since 1998, and the questions that reach us are rarely about the headline number. They are about the basis it was produced on. Our sanitary pads supplier page sets out the basis we quote on, the wholesale buying guide covers the rest of the document set, and the factory capability page shows which lines can hold which profiles. Changing the polymer loading or the profile moves the minimum order quantity, since a new core means new tooling before it means new material, and the FAQ covers the versions of this question we get most often. Send the basis line from your current brief and we will tell you which of the two figures it was produced from.
Frequently Asked Questions
Why is sanitary pad absorption capacity ml not a measured volume?
Because the bench weighs the sample before and after uptake and divides the gain by an assumed liquid density. The millilitre figure is that division, so it inherits every assumption standing behind the density, including the one about which liquid was used.
Can a millilitre figure be converted into grams of polymer?
No, the two describe different things. One states how much liquid the core accepted in a test, the other states how much absorbent was dosed on the line. The bridge between them is a tested ratio, and that ratio changes when the core profile changes.
Why does a thinner core report a lower figure at the same loading?
Compaction removes the air the polymer needs in order to swell, so less liquid fits before the surface stays wet. The polymer mass has not changed at all, which is exactly why the two numbers move apart when the profile is tightened.
What happens to the dosed polymer before it reaches the pad?
Part of it never lands. Some leaves with the exhaust air, some coats the duct, and some separates from the fibre inside the air stream. The loading in the finished pad is therefore lower than the feeder setting, and the gap drifts during a run.
How should two quotes in different units be compared?
Ask both suppliers for the polymer mass per pad and for the volume the core accepted in their test, then divide the second figure by the first. The ratio that comes out has no unit and can be compared directly, whoever produced it.
