The Additive Moves, the Resistance Follows
Two mechanisms behind a drifting reading
Conductive and dissipative foam get their electrical behaviour from additives: carbon black in a conductive compound, or a hygroscopic antistatic agent in a dissipative one. Carbon-loaded compounds are comparatively stable because the filler is part of the polymer matrix. Antistatic agents are not — they are designed to migrate to the surface, where a thin layer of moisture-loving molecules holds enough water to bleed charge away.
Migration is the feature, not a defect, but it has a consequence: the surface layer can be rubbed off, washed off, or driven back into the bulk by heat. A part that measured 10^6 ohms on receipt can read 10^10 ohms after a year in a dry store, not because anything was mislabelled, but because the surface layer is gone or the ambient humidity collapsed.
What to verify before filing a complaint
Re-test under controlled conditions first: 12 percent relative humidity is not a fair test of a hygroscopic system, and neither is a quick probe on a dusty surface. Condition the sample, clean the contact area, and use the same electrode geometry as the original qualification. If the reading recovers, the material did not change — the measurement conditions did.
If the reading does not recover, check the storage history against the shelf-life statement and look at how parts were handled. Wiping with solvent, repeated handling in a gloved cell, and stacking under direct sunlight all shorten the working life of an antistatic surface. Where drift matters to the process, qualify with a re-test interval rather than a one-time certificate, and keep the interval in the incoming inspection plan.
FAQ
Why does ESD foam resistance increase over time?
The antistatic surface layer can be worn, washed or dried back into the bulk, and low ambient humidity reduces the moisture film that carries charge. Both raise the measured resistance without any change in the compound.
Is a drifting reading a quality defect?
Not necessarily. Antistatic systems are humidity dependent and surface dependent by design. Confirm with a conditioned re-test under the original qualification conditions before treating it as a defect.
Should ESD foam be re-qualified periodically?
Where the foam is part of a controlled process, yes. Set a re-test interval in the incoming inspection plan rather than relying on a single certificate from the first batch.
What storage conditions slow the drift?
Sealed, out of direct sunlight, away from heat sources, and within the shelf-life window stated on the data sheet. Do not wipe with solvent, which strips the surface layer.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.