Somebody specifies antistatic spools, the certificate says antistatic, and static problems continue. The gap is usually that nobody has ever checked whether the property is present, how strong it is, or whether it is still there after a year of service.
What the property actually is
Standard polypropylene is an excellent insulator. Charge generated by friction stays where it is, which is why ordinary plastic attracts dust and why static discharge happens. Antistatic and conductive grades are modified so charge can dissipate rather than accumulate.
The property is expressed as surface or volume resistivity, measured in ohms — or more often as ohms per square for a surface measurement. Lower means more dissipative. The categories commonly used are roughly: conductive at the lowest, dissipative in the middle, insulative at the top.
Our article on when resistivity matters covers whether you need it at all. This one is about verifying it.
What you can check without a laboratory
A handheld surface resistance meter. The proper tool, and they are not expensive. Two probes placed on the surface at a defined spacing, a reading in ohms. You need to know the probe geometry to convert to ohms per square, and reputable instruments do it for you.
A simple continuity check will tell you almost nothing useful. It distinguishes conductive from insulative at the extremes and misses the dissipative range entirely, which is where most antistatic material sits.
The pragmatic field test — does it hold dust? Rub the spool, hold it near small particles or light offcuts, and see what happens. Crude, subjective, and genuinely informative as a comparison between two spools. It will not give you a number and it will not settle a dispute.
What affects the reading
This is where people get misleading results:
- Humidity. Surface resistivity is strongly affected by ambient humidity. Many antistatic mechanisms depend on moisture in the air. A reading in a dry heated building in winter can be very different from the same spool in August.
- Surface contamination. Dust, release agent, product residue, fingerprint grease — all change the surface measurement.
- Age and cleaning. Some antistatic additives work by migrating to the surface, and they can be depleted by repeated washing and by time. This is the reason our cleaning article advises caution about methods.
- Where you measure. Readings vary across a moulded or fabricated part.
How to do it usefully
- Measure several points and record the range rather than a single figure
- Record humidity and temperature alongside, or your numbers are not comparable over time
- Clean the surface first in a standard way, and use the same method every time
- Compare against a known reference — an untreated spool measured the same way gives you the contrast you need
- Measure new spools and re-measure after service. The change over time is more informative than the absolute value.
What a specification should say
Vague requirements produce vague results. Worth stating:
- The resistivity range you need, and whether surface or volume
- Measured under what conditions — humidity and temperature matter enough to specify
- Whether it must survive washing and how many cycles
- Whether it is required after service or only when new
That last one is rarely asked and often the real question. If you need the property for the life of the spool, that is a different specification from needing it on delivery.
What does not follow from a reading
A resistivity figure tells you how quickly charge dissipates. It does not tell you:
- Whether your static problem will be solved — the generation rate matters as much as the dissipation
- Anything about ignition risk in a hazardous area, which is a specialist assessment and not something a handheld meter settles
- Anything about the product being wound, which may be the thing actually generating the charge
Our position
We can tell you what grade our hollow board spools are made from and what the material supplier states for its resistivity, under what test conditions. We will not quote a performance figure we have not measured, and we will tell you when we think your static problem is not going to be fixed by changing the spool — which is often the case.
If you want to test, buy a meter. It is a modest purchase and it turns a claim into a measurement.