Wiper testing has a vocabulary problem. The same word — "particles" — describes three different measurements, and the numbers are not interchangeable. IEST-RP-CC004, the recommended practice for evaluating wiping materials, defines the methods that most suppliers cite. Knowing what each one does is the difference between a comparison and a coincidence.
The biaxial shake test
A sample is placed in a container with a liquid, shaken in two axes for a defined period, and the liquid is counted. The result is a particle count attributable to the wipe under mechanical agitation.
What it measures: shedding under a repeatable mechanical stress. What it does not measure: what happens when the wipe is dragged across a surface, folded, or used with a solvent. The shake test is a screening tool, not a simulation of cleaning.
Wet and dry particle release
Some methods run the wipe dry, others wet. Dry testing tends to produce higher counts because fibres become airborne and static; wet testing captures particles that transfer into a liquid medium. Neither is more correct — they answer different questions.
If your process is a dry wipe, ask for dry data. If it is a solvent wipe, ask for wet data with the same solvent. A supplier who only has one number has only answered one question.
Fibre release
Fibre counting is usually done by filtering the test liquid and examining the filter under magnification. The result is reported as fibres per unit area or per wipe, with a counting rule that defines the minimum length and width of a countable fibre.
The counting rule matters. A report that counts only fibres above 100 µm will look far better than one that counts above 10 µm, even for the same wipe. Ask for the rule before comparing.
| Test | Reports | Best used for | Blind spot |
|---|---|---|---|
| Biaxial shake | Particles in liquid | Screening, lot consistency | Surface contact |
| Dry release | Airborne/loose particles | Dry wiping, cleanroom entry | Solvent behaviour |
| Fibre count | Fibres per area | Edge and fabric quality | Sub-visible fibres |
| NVR / extractables | Residue mass | Critical surfaces, coatings | Which compounds |
| Sorption | Liquid uptake and rate | Task sizing | Real solvents |
Sorption and uptake
Sorption tests measure how much liquid a wipe takes up and how quickly. They are the most directly useful tests for sizing a task, and the least quoted in marketing material, because a high uptake number is not as impressive as a low particle count.
Uptake is usually reported as a multiple of the wipe's dry mass, or as millilitres per gram. The two are not directly comparable across materials. Ask for the method and the liquid.
Two suppliers can report "< 5 fibres" and mean different things. The method, magnification and counting rule are the number.
Extractables and non-volatile residue
Extractables testing soaks the wipe in a solvent, filters it, evaporates the filtrate and weighs what remains. The result is non-volatile residue, usually in milligrams per gram of wipe or per unit area.
This is the test most closely tied to real cleaning problems on coated or sealed surfaces. It is also the test most sensitive to sample preparation: gloves, tweezers, the cleanliness of the glassware and the grade of solvent all influence the result.
Ask for the solvent and the soak time. A water extract and an IPA extract of the same wipe can differ by an order of magnitude, and both can be true.
What the standard does not do
It does not set pass/fail limits. It describes methods so that results can be compared. The limits are yours to set, from the class, the surface and the product risk.
It also does not simulate use. No recommended practice can capture the pressure, motion and solvent combination of your process. That is why a bench test with your own technique remains the most useful qualification step after the paperwork is done.
Which test should I ask for first?
Particle and fibre release by a named method, then extractables if the surface is critical. Sorption is useful for sizing but rarely decisive in qualification.
Can results from different laboratories be compared?
Only if the method, sample preparation and counting rules match. Otherwise treat them as separate measurements rather than a ranking.
Does IEST-RP-CC004 apply to swabs?
It is written for wiping materials. Swab testing has its own practices, though the principles of particle, fibre and extractable measurement carry over.
The value of the standard is not the certificate it produces. It is the discipline of asking what was measured, how, and under what conditions — three questions that turn a datasheet into evidence.
