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Converters of non-conductive web surfaces, such as pressure-sensitive adhesive (PSA) tapes, must be vigilant in controlling static electricity. High levels of electrostatic charges can be dangerous; moderate levels can permanently destroy an adhesive system, foul and jam production processes, and damage finished products.
New technology allowing faster production speeds exacerbates static-related problems. To reduce static, operators sometimes will intentionally slow down the production process, which can be a costly practice.
THE CAUSE: CONTACT AND SEPARATION
Static electricity is an accumulation of electrical charges on a surface. The polarity of the charge can result from an excess of electrons (negative charge) or a deficiency of electrons (positive charge). Charges accumulate on the surface of nonconductive material, such as paper or plastic film, and can also accumulate on ungrounded conductive materials, such as machine parts and the human body.
When surfaces are in contract with each other, a transfer of electrons occurs. Friction, pressure and web speed accelerate this transfer. When separated, the surface that has gained electrons becomes negatively charged, and the mating surface giving up the electrons becomes positively charged.
This process, known as triboelectrification (or tribocharging), results in the generation of static charges on the surface of PSA tape as it unwinds from the roll and as it contacts and separates from surfaces, such as idler rolls, nip roils, and printing or coating rolls. The buildup of electrostatic charges is a cumulative process, increasing each time the web contacts another surface. PSA tape can, therefore, accumulate a high charge as it passes through transport systems in which it comes in contact with several idlers.
On non-conductive materials, such as papers and films used for PSA tape, charges do not build up uniformly across the surface. A single, contiguous surface may accumulate a positive charge in some spots and a negative charge, or no charge, in others. The charge's intensity can also vary dramatically. For example, a positively charged...