Monroe Electronics Electrostatic Fieldmeter - Static Monitor - model 177A - Manual (Page 57)

APNE-0015 56
Factors That Most Affect Triboelectric Charging
Contact Pressure: Increased pressure increases contact area by reducing the air gap
between surfaces.
Contact Time: Increased contact time can increase charge transfer by enlarging the contact
area.
Draining Velocity: The speed at which charges travel along the web toward recombination.
It is relatively constant and independent of parting velocity, but is directly related to web
front/back surface and bulk conductivity. Net charge builds on a web when parting velocity
is greater than draining velocity.
Electric Field: The electric field generated at the location where the surfaces separate may
provide enough force to affect the return of charges to their original surfaces.
Humidity: As humidity is increased, generally material conductivity increases, and charge
accumulation decreases due to charge backflow.
Particulate Contamination: Dust and other particulates on surface layers affect surface
chemistry and contact area, which affect charging.
Parting Velocity or Separation Speed: Affects the time electrons have available for returning
to their original surface. Generally, the faster the surfaces are separated the more charge is
left on the surfaces.
Slip (or Slide): Causes frictional charging due to relative motion between surfaces. This can
also cause transfer of actual material from one surface to the other, which will affect
charging as well.
Surface Hardness: Soft surfaces make contact that is more intimate and tend to cause
more charging during separation.
Surface Layer Chemistry: Affects how charging takes place when surfaces of differing work
functions contact each other.
Surface Roughness: Charge transfer decreases as surface roughness increases due to
less contact area between the surfaces.
Temperature: Elevated surface temperatures usually result in increased charge transfer
due to higher molecular energy levels as well as to decreased surface moisture films.
Locations Of Charge Transfer In Web Converting Machines
When the web is wound into roll form, the face and back are in contact. So charge transfer
can occur during unwinding (see Figure IV-1 for example). Surface chemistry and intimacy
of contact are important factors here. Sometimes "charge balancing" is attempted by
chemists who add surfactants and other chemicals to the surfaces to control front-to-back
charging. In addition, the more intimate (greater) the contact between layers, the more
charge is transferred. Intimacy is influenced by such factors as surface hardness,
roughness, thickness, force concentration, and winding tension. Roll history (moisture
changes, temperature changes, winding tension) can be an important factor because it may
affect conductivity or contact area.
Corona discharge treatment (CDT) is applied to the web in some converting processes.
CDT treatment is used to increase the web’s ability to accept coating or printing layers.
Because CDT purposely generates large quantities of ions to alter the web’s physical
characteristics, high charges are also left on the web, which then must be reduced or
eliminated.