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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.
David Swenson of 3M 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. The "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, went 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. The generation of surface
charge imbalance during unspooling was therefor not unexpected, and is a
common problem in this industry.
On entering the factory floor and far from the equipment, a 200KV/ft
handheld electrometer was found to slam to full scale. When Mr. Swenson
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 caught
in the web.
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
A thought: unspooling of film typically generates higher net charge on
the film than on the spool. The unequal imbalance indicates that charge
is being lost through some unseen path, most probably as ionized air.
So, if the charged "tent" of film is negative in the above situation,
and if large quantities of positively charged air are being generated,
then perhaps the "invisible wall" 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" which can trap flies. - B.B.
Reference: Article about the "Wall" in ESD Journal
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 ions 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 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.
Speculative, untried experiments:
CAN FULL OF AIR: Generate a high-density cloud of negative air ions.
Place a large positive potential upon a metal can or bucket. Direct
the air ions into the bucket. This should form a charged capacitor,
with the air being one plate and the bucket the other. Ground the
bucket and see if the contained ionized air is affected. (Once the
ion/metal capacitor effect has commenced, you should be able to remove
the power supply without destroying the effect.) Drop small, light
objects into the bucket to test for pressure-gradient repulsion
forces. Inject smoke into the bucket to observe possible "ion wall"
effects. Connect the bucket to a large negative potential and observe
whether the contained ionized air is disrupted.
ION COATED WIRE: Hang a long piece of thin wire from a source of large
positive potential, so the wire swings freely like a pendulum. Bathe
the wire in the output of a negative ion generator. Remove the ion
generator and bring a neutral object near the wire to see if a
transparent "ion shell" repels the wire from the object via its pressure
gradient. Swing the wire as a pendulum, see if it swings slower because
of the extra mass of entrained air. Remove the positive potential from
the wire and test to see if the repulsion vanishes (it might not, if
the air/metal capacitor is not disrupted!)
AIR-THREADS: Connect a single needle to the positive terminal of a
source of large potential, so that ion-wind is generated. Bath the
needle in the output of a negative-ion generator. Use a Schlerien
optical system to observe the needle and its surroundings. The
positive ion-wind from the needle will attract a surrounding negative
sheath and build a "thread" structure of oppositely charged air.
The pressure gradients within the "thread" will be observable in the
Schlerien image as optical distortions, and an image of a long
thread-like structure will appear. The "thread" will be found
to be of neutral polarity. Try building another "thread" generator,
but of opposite polarity. What happens when "threads" of opposite
polarity interact? Try injecting smoke or water-mist into the
thread, and see if it can be made directly visible. Try immersing the
thread-generator within a pool of CO2 mist and see if an upwards-
directed thread will entrain and transport mist along with it. The
"Thread" phenomena was discovered by C. Yost, but the (possible)
involvement of opposite ions is my untested speculation.
ALIEN SPACECRAFT: Build a low-mass disk-shaped device having negative
ion generators around its rim and a large central foil disk on the
bottom which is insulated from the rest of the device. Connect power
supplies to the ion generators via an umbilical. Connect a large
positive potential to the central plate. A region of negative ions
will form adjacent to the positive plate, and a pressure gradient
will appear. If the total weight of the device is low enough, the
device will lift into the air and hover on top of its transparent
ion-bubble. If a large enough device could be built, the power supplies
could be placed onboard!
"THICK" AIR: Suppose a volume of air was entirely composed of equal
numbers of ions of opposite charge. They would attract each other and
might form a low-density semisolid, like a cross between aerogel and
ionic crystal (such as sodium chloride.) Suppose a 3-dimensional array
of small volumes of ionized air could be produced. Volumes of opposite
polarity would experience attraction forces and the array as a whole
would be neutral, and would be at higher pressure than ordinary air.
The array might act like a sort of invisible "solid" which would cohere
together, resist disruption, eject intruding objects, and would move as
a unit when pushed. It would act somewhat like an invisible semi-solid
mass. The organized-ion air could take the form of positive
threads in a negative medium, positive spheres in a negative medium,
alternating layers of positive and negative, various close-packing
arrays of opposite polarity, etc. A gas composed mostly of positive and
negative air ions (few neutral molecules) might exhibit a behavior very
different than ordinary air. It would exhibit increased pressure,
density, and refractive index. It would be heavier than normal air, and
might be directly visible as an area of optical distortion in the air.
The increased attraction between ions might raise the boiling and
freezing temperatures to anomalously high values.
BURST A BALLOON: Fill a large plastic bag with air, connect the two
ends of a hollow plastic pipe to the bag, then install a small fan and
an ion generator inside the pipe. When the ion generator is operated,
the ionized air within the bag will self-repel and exert force upon the
walls of the bag. The pressure should increase linearly as long as the
ionizer is operating, and the bag should burst from electrical pressure.
DIPOLE AIR: Build two of the above 'bag' devices, but having opposite
polarities. When operated, the attraction forces between them should
grow to a large value, until the bags tear loose from their moorings
and slam together.
ELECTRIC DIRIGIBLE: Fill a hot-air balloon with air, then install a
negative ionizer and let it run until the contents of the balloon become
highly charged. The balloon should experience a lifting force as the
positively charged sky attracts it and the negatively charged earth
repels it. Is the force too feeble to lift a dirigible? Then try
charging soap bubbles?
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