"Invisible Electrostatic Wall" at 3M adhesive tape plant
SESSION 7: SPECIAL SESSION, 17th Annual EOS/ESD Symposium
THURSDAY, SEPTEMBER 14, 1995, 8:00 am
SESSION 7: SPECIAL SESSION: ELECTROSTATIC CONSIDERATIONS IN INDUSTRY
MODERATOR: D. Swenson, 3M
7.7 CASE STUDY - LARGE PLASTIC WEB ELECTROSTATIC PROBLEMS, RESULTS AND
CURE, D. Swenson, 3M Company
Tremendous static charge generation on a plastic web causes unique
physical phenomena and special problems. Solution was simple and cost
effective.
Also see: article in the ESD Journal
David Swenson of 3M Corporation describes an anomaly where workers encountered a strange "invisible wall" in the area under a fast-moving sheet of electrically charged polypropelene film in a factory. This "invisible wall" was strong enough to prevent humans from passing through. A person near this "wall" was unable to turn, and so had to walk backwards to retreat from it.
This occurred in late summer in South Carolina, in extremely high humidity. Polypropelene (PP) film on 50K ft. rolls 20ft wide was being slit and transferred to multiple smaller spools. The film was taken off the main roll at high speed, flowed upwards 20ft to overhead rollers, passed horizontally 20ft and then downwards to the slitting device, where it was spooled onto shorter rolls. The whole operation formed a cubical shaped tent, with two walls and a ceiling approximately 20ft square. The spools ran at 1000ft/min, or about 10MPH. The PP film had been manufactured with dissimilar surface structure on opposing faces. Contact electrification can occur even in similar materials if the surface textures or micro-structures are significantly different. The generation of a large imbalance of electrical surface-charge during unspooling was therefor not unexpected, and is a common problem in this industry. "Static cling" in the megavolt range!
On entering the factory floor and far from the equipment, Mr. Swenson's 200KV/ft handheld electrometer was found to slam to full scale. When he attempted to walk through the corridor formed by the moving film, he was stopped about half way through by an "invisible wall." He could lean all his weight forward but was unable to pass. He observed a fly get pulled into the charged, moving plastic, and speculates that the e-fields might have been strong enough to suck in birds!
The production manager did not believe Mr. Swenson's report of the strange phenomena. When they both returned to the factory floor, they found that the "wall" was no longer there. But the production workers had noticed the effect as occurring early in the morning when humidity was lower, so they agreed to try again another day. The second attempt was successful, and early in the morning the field underneath the "tent" was strong enough to raise even the short, curly hair of the production manager. The "invisible wall" effect had returned. He commented that he "didn't know whether to fix it or sell tickets."
- Bill Beaty
Problems: coulomb forces would be expected to *attract* a person into the "chamber" formed by the PP film, and the attractive force should be fairly linear across distance. There should be no "wall" in the center, a "wall" is repulsive and nonlinear.
If for some reason a person was repelled from the center of the chamber rather than being attracted, there still should be no "wall," since the repulsion force should exist over a large distance; it should act like a deep pillow which exerts more and more force as one moves deeper into it. It should not behave like a "wall". This is how magnets and iron behave, and this is how e-fields and conductive objects should also behave.
A thought: unspooling of film typically generates higher net charge on the long piece of film than on the limited surface of the spool. However, since net charge is conserved, imbalances of charge MUST be equal and opposite. The charge on the entire length of moving film MUST be equal in magnitude to the charge on the spool, yet the charge on the film is very large and is continuously increasing. The limited surface-charge on the spool indicates that opposite charge is being lost through some unseen path, most probably as IONIZED AIR.
Charged air would arise in the cleft between film and spool as the film was peeled from the spool. I wonder if film was peeled from the top of the spool, so that any ionized air would be launched into the "tent-chamber" region? If it was peeled from the bottom of the spool, the charged air would end up outside the "tent." Or, if a corona discharge arises in the cleft between film and spool, perhaps the UV and e-fields of this corona can ionize the air on both sides of the exiting plastic film, and spray the charged air everywhere.
So, if the charged "tent" of film is negative in the above situation, and if a large quantity of positively charged air is being generated at the spool, then perhaps the "invisible wall" is caused by a cloud of suspended air ions. Perhaps it is a pressure gradient created by ionized air trapped under the tent by electrostatic attraction. Yet this effect would be expected to create a diffuse zone of increasing force, not a "wall", but an "invisible pillow."
However, a volume of charged air is somewhat analogous to iron filings near a magnet. If a solid sheet of iron filings is held in place by a magnet, then a literal "wall" is created, and this wall will resist penetration by nonferrous objects. If in the above manufacturing plant a sheet of highly charged air is for some reason being held in place by the fields created by the charged film, then a transparent "wall" made of charged air would come into being. It might resist penetration by human bodies.
My question is this: if the entire situation could be turned on its side, so the "invisible wall" became an "invisible floor", could a person *stand* on it? Have we discovered the long-sought "Zero-G waterbed?"
- B.B.
Reference: Article about the "Wall" in ESD Journal (IT'S BACK! 8/2000)
MORE THOUGHTS:
From: Beaty, William J
Subject: Ion cushion
Date: Monday, August 12, 1996 4:02PM
Also: I wonder if the (I assume) huge quantity of air ions had anything to
do with your weird phenomenon. Maybe the "wall" effect involves a plug of
ionized air which is held in place by the opposite charge on the film. If
so, your repulsion phenomenon would not occur if the "tent" of film was
replaced with highly charged metal plates, since the source of
oppositely-polarized electric wind would then be missing. I'm still
convinced that the charged film should produce an attractive force upon a
human body. Repulsion requires that the human be charged with the same
polarity as the PP film, yet induction should produce an *opposite* body
charge, so attraction is expected. But if a plug of oppositely-charged
air was strongly attracted into the "tent" of PP film, it might produce a
significant pressure-gradient in the surrounding air. A fraction of a PSI
per foot would be more than enough to prevent someone from walking
forward. If this is the origin of the effect, then the repulsion forces
you experienced involved air pressure rather than electrostatic
attraction/repulsion.
This might be an entirely new way to accomplish levitation. Attract
a whirling blob of ionized air to an oppositely-charged plate, then use
the resulting pressure gradient to lift and manipulate uncharged objects.
Sort of like a fluidized bed, but using charged air instead of sand.
Why doesn't the population of opposite ions "plate" itself onto the
plastic surface? Maybe it tries to do so, but the air within the moving
tent is swirling like a horizontal tornado, so the charged air cannot
simply move straight to the plastic film. If true, then the phenomena
would not appear if motionless charged air and oppositely charged plastic
were present. The tent shape and the motion of the plastic would also be
required. Incredible coincidence that all the required components could
ever come together in one place! (if this is indeed how it works!)
