Static electricity is an ever-present issue for label printers and converters. As many know from experience, static can cause detrimental effects to a print run, resulting in costly quality problems. The phenomenon also presents a safety issue that can cause electrostatic shock to personnel. It can even ignite vapors in solvent-based inks and coatings.

What problems are caused by static electricity?
Three significant problems caused by static electricity in the workplace include electrostatic attraction and repulsion, shocks to operators, and fire risk. Electrostatic attraction/repulsion is one of the most widespread issues in the plastics, packaging and paper industries and can result in product issues such as sticking, repelling each other, and attracting dust. All of these elements ultimately lead to loss of productivity and profits. Shocks to operators, although not fatal, are becoming increasingly important as health and safety issues gain prominence and can result in operator errors. Fire risk is critically important in industries where solvent materials are used.  Static charge on material can cause a spark discharge which can ignite the solvent and create a fire.

There are additional dangers of static build-up with pressure sensitive labels used in digital presses, too. When labels are processed through digital laser printers, they are charged through an electrostatic process that causes print toner to adhere to the label surface. When printed labels are peeled, that action can generate static charge (tribocharging), which has the potential to ignite vapors in industrial environments in which chemicals, solvents and flammable materials are present.

In 2017, an employee working at a global paint and coating manufacturer was filling 5-gallon pails of primer using an automated filling device. As pails were filled, the employee sealed the lids and applied a vinyl self-adhesive label to each pail, discarding the label backing into a wheeled trash bin nearby. While mopping the floor with a cleaning solution, the employee contacted the wheel of the cart with the mop head, generating a static spark that jumped from the wheel and ignited the solvent in the cleaning solution, setting the mop head on fire.

Although the fire was quickly extinguished, an investigation later determined that the voltage from the wheeled trash bin was generated by the electrostatically charged label backings it contained.

Corrective action
After making this determination, the paint manufacturer immediately engaged its label management vendor and an independent testing center to identify the label stocks that would generate the lowest amount of electrostatic voltage during label application.

The company turned to a process safety management firm that helps businesses in preventing explosions, fires and accidental material releases, to conduct static testing of PPG Teslin substrate, vinyl and PET label materials.

First, a label peel test was conducted to determine the electrostatic charge build up when an electrically isolated label set is peeled off from its backing and placed in a Faraday Cage under different conditions of relative humidity. The specific charge density calculated was then compared with maximum surface charge density (charge per unit area of 2.7×10-5 Coulomb/m2) in free space.

When peeled from their backings, vinyl labels can become electrostatically charged to 29% of the maximum charge density possible (2.7×10-5 C/m2) at ambient humidity environments, and PET labels can become charged to 30%. The same test of Teslin labels showed that they only become electrostatically charged to 0.065%. The charge on both the vinyl and PET labels can give rise to incendive electrostatic discharges under ambient and low relative humidity conditions.

This test was performed in accordance with ASTM D257 and conforms to the ISO 17025:2017 standard.

Surface resistivity test
A second test comparing charge relaxation (decay) time of Teslin substrate, vinyl and PET was conducted to measure the time needed to dissipate the electrostatic charge from the surface of the materials to ground (earth).

At 71% relative humidity, in less than 0.5 seconds Teslin substrate dissipated more than 90% of its starting charge of 5000V.  Both PET and vinyl held approximately 100% of their starting charge of 5000V over the same amount of time and showed minimal tendency to dissipate their charge. Although vinyl eventually dissipates its charge, it takes more than 3 seconds and does not meet the static dissipative definition of an electrostatic material.

This test was performed in accordance with MIL-STD-3010C, test method 4046.

Conclusion
Based on the results of the testing, the paint manufacturer selected Teslin labelstock for its ability to dissipate static and accommodate laser printing. The third-party testing supported the safety benefits gained by switching to Teslin labels, and showed that vinyl and PET labels are highly chargeable and pose a significant safety threat. By switching to Teslin labelstock, the company increased worker safety and diminished the risk of fire by reducing static during label-peeling and application in its production facilities.