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Question We coat photographic film on polyester. The photographic layer is ESD sensitive. That is, we have some problems with discharge when unrolling our film. We also have transport problems (clinging) in dry climates. Both are serious issues of us. I have been told that measuring the surface resistivity of our products and trying to decrease it by modifying our formulae will reduce these problems. Does this sound correct? Would an ESD Association Standard 11.11 type resistivity meter be appropriate? Do instruments that would measure field decay rate apply here? Also, can the properties of the polyester base corrupt or influence the results of our measurement if it does not touch anything (if it is covered by the coated layer)? There are polyesters called “antistatic” but I think they are just dissipative. -Bryan, Glen Cove, New York
Answer Recommendation would be to employ a good air Ionizer to minimize the charge generation problem.Making the film more conductive will only help if the film is also in contact with a ground path to neutralize the charge imbalance. This sounds like a material problem, where two dissimilar materials that come in contact and then separate will leave a charge imbalance on each other. Triboelectric generation is one mechanism to generate charges on materials. This charge imbalance will become larger the farther away the materials are in relative position in the triboelectric series chart. In lieu of changing materials to minimize this phenomena or controlling the relative humidity, which may help a little, the most effective method is ionization. Using an ESD Ionizer, one that outputs balanced air ions to neutralize charged surfaces, especially insulative surfaces which cannot be grounded. The property of a material to conduct current is important in ESD Control. We have 3 regions in resistance that we classify materials in. The most conductive is in the region called ‘conductive’ or less than 1x104 ohms. Then next and more ideal range for ESD control is the ‘dissipative’ region or a material with a surface resistance from 1x104 to less than 1x1011 ohms. The last region is a non-conductive one or insulative one and is greater than 1x1011 ohms. You can measure a surfaces resistance with a good megohmeter. S11.11 is useful for classifying packaging materials or measuring surface resistivity. Standard ESD S4.1 may be more practical for making general surface resistance measurements. The reason the dissipative range is considered ideal is that when a charged device comes into contact with a neutral dissipative material, the corresponding charge transfer (current) will be a controlled bleed and not an electrostatic discharge. Another property that is preferred in controlling ESD is one that use to be called ‘antistatic’, now called low tribocharging to help minimize confusion. When two materials come into contact and separate, how much charge they leave on each other can be classified by how antistatic they are. It is generally accepted that a ‘Low Tribocharging’ (formerly known as antistatic) material is one which does not tribocharge over 200 volts (depending on the referring standard - this case EIA-625). So, an antistatic material may be preferable in your application.
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