"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."
===
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)
Source:
http://amasci.com/weird/unusual/e-wall.html