Converters of non-conductive web surfaces, such as pressure sensitive adhesive (PSA) tapes, need to vigilantly control static electricity. High levels of electrostatic charges can be dangerous, and even moderate levels can permanently destroy an adhesive system, cause fouling and jamming of production processes, and damage to finished products. New technology allowing faster production speeds exacerbates static-related problems.
Static electricity is an accumulation of electrical charges on a surface. As surfaces are in contact with each other, a transfer of electrons occurs. Friction, pressure and web speed will accelerate this transfer. When separated, the surface which 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 (Pressure Sensitive Adhesive) tape as it unwinds from the roll and as it contacts and separates from surfaces such as idler rolls, nip rolls and printing or coating rolls. The build-up of electrostatic charges is a cumulative process, increasing each time the web contacts another surface, thereby accumulating a high charge as it passes through transport systems where it comes in contact with several idlers.
When disrupted by static, the silicone coating will no longer function as a release system in the area affected by the discharge, making it very difficult to remove the liner as designed. The accumulation of electrostatic charges can also cause sheets of materials to stick together, creating jams in downstream processes. Electrostatic fields attract particles, resulting in surface contamination, quality problems in printing, coating and laminating, as well as cleanliness problems with medical PSA applications. Moreover, static charges can cause uneven coatings and "wicking" of inks, and pressure-sensitive tape carrying a static charge can damage sensitive electronic components.
The papers and films used for PSA tape do not build up charges uniformly across the surface; they accumulate a positive charge in some regions and a negative charge, or no charge, in others, with varying intensity.
To control static on coating and converting machines, two types of ionizers are available: active and passive. Active ionizers, also called permanent static control devices — such as high output static neutralizer bars and static blowers — are powered by an external source of electrical voltage. Alternatively, radioactive material can be used as an ion source. The relatively high purchase cost of active ionizers has limited their use in the converting industry.
Passive ionizers, also called induction ionizers and non-permanent static control devices, include static tinsel and static string, which are simply grounded emitters placed parallel and close to the charged material. The electrical energy of the charged material will excite the passive ionizer, causing it to generate air ions of the opposite polarity. If properly positioned, a passive ionizer can successfully reduce the electrostatic charge.
Tinsel, also know as garland, is relatively inexpensive and remains one of the most widely-used methods for controlling static. However, anti-static tinsel is composed of copper, and pieces of this soft metal can break off and contaminate the product. To address this problem, engineers at Adchem Corporation tested a variety of passive ionizer products to determine whether any of these methods could provide equal or better static control compared to tinsel, without tinsel's risk of product contamination. Side-by-side evaluations were performed at Adchem's 200,000 square foot engineering and manufacturing facility in Riverhead, New York. Five different passive ionizer products were tested: anti-static tinsel, static elastic, static wire and two types of static string. Each anti-static product was prepared and then installed one after another in static "hot-spots" along the production line. Precise measurement of the static charge over a free-span web surface was performed with a hand-held electrostatic field meter, repeated for five different PSA products with non-conductive web surfaces composed of a variety of materials.
Test results showed that alternative passive ionizer methods can provide better static control than tinsel. In fact, Static String from Stop Static, a division of Alpha Innovation of Marblehead, MA, provided significantly better static control for all five web surfaces tested. The Static String product consistently maintained static levels below the "5000 volts rule" established by Albert E. Seaver in 1993 and generally accepted as the industry safety standard. The Static String product from Alpha Innovation outperformed other passive ionizer products including a similar string product from another manufacturer.
An advantage of the Static String product is that it is effective when installed 0.25 to 2.0 inches away from the web. Not needing to make contact with the web lessens the possibility of web damage, coating damage or contamination. In addition, the product was found to be relatively easy to install and remove, especially compared to tinsel.
Tests at Adchem showed that Static String, installed correctly, consistently reduced electric charge below the desired level of 5kV in all products tested to date. Adchem has replaced tinsel with Static String on all its coating and converting machines, resulting in the measured static level on all machines being reduced. The Adchem plant has thus eliminated the risk of product contamination by pieces of copper tinsel, therefore guaranteeing a higher level of quality control.
Contact: Adchem Corporation, 1852 Old Country Road, Riverhead,NY 11901 631-727-6000 fax: 631-727-6010 E-mail: firstname.lastname@example.org