Substrate Static and Surface Energy Demystified
Many screen-printed products are insulators, so completely eliminating static through grounding isn’t really possible.
Static is the excess or deficit of electrons on a material. Static in screen printing is most often caused by friction, separation, heat change, and improper grounding. Screen-printed products normally experience all of these during handling. Potential results of untreated static energy include ink spidering, sheets sticking together, delivery/stacking difficulty, accumulation of dirt and dust, electric shocks, damage and interference in electronic components, and even fires and explosions.
Friction: Static is most often caused by friction, the rubbing of two materials together during the printing or converting process. Rubbing a plastic substrate over a metal feedboard creates a charge. The plastic picks up the negative charge, while the metal feedboard carries the positive charge. Metal is a good conductor, so its acquired charge is short-lived. Plastic is a poor conductor, so it retains its charge for a long time.
Separation: Static can be generated when two materials are separated―for example, pulling the barrier film off of a substrate. Generally, the barrier film holds a charge that is opposite of the product it protects. Media that holds a charge generated by separation can attract airborne dust and debris from several feet away.
Heat change: Products that are subjected to continuous heat changes can build a static charge. Dryers are a major source of this phenomenon. Charges and temperatures constantly change when a printed product is in the drying phase. In extreme conditions, dust and particulates can be drawn onto the product’s surface and adhere to the product.
Improper grounding: Even the greatest efforts to ground machinery can be thwarted by greased bearings and poor metal-to-metal connections, both of which can actually interrupt the ground. Furthermore, some charges may be too high to ground properly. Many screen-printed products are insulators, so completely eliminating static through grounding isn’t really possible.
Active static-elimination systems include active electrical ionizers, static bars, air ionizers, ionizing nozzles, steady-state DC generators, and pulsed DC controllers. They create ionization by forcing high voltage to ground and breaking down the air molecules in between into low levels of ozone. Passive static control uses grounding and conductive metals. Carbon fiber, stainless steel, phosphor bronze, and copper tinsel are commonly used in screen printing.
Ink adhesion and print quality on plastic films can suffer when ink doesn’t properly wet, or make maximum contact with, the substrate’s surface. Optimal wetting relies on the surface tension of the ink being equal to or less than the critical surface tension (named as such because solids do not have a surface tension in the same sense that liquids do) of the substrate―a contact angle of 0°.
A liquid that is under stress will increase its surface area; after the stress is removed, the liquid will spring back to minimize its contact area, thereby producing a contact angle. Solids are unable to spring back in reaction to surface tension. Pretreatments can alter a plastic substrate’s critical surface tension.
Flame and corona treatments are two common pre-print processes that elevate the surface energy of plastic substrates. Both techniques effectively oxidize a very thin layer (0.0005-0.001 micron) of the substrate’s surface. Flame treatment briefly exposes the substrate’s surface to energized particles created in a combusted mixture of fuel gas and air.
The process affects the distribution and density of electrons on the substrate’s surface and polarizes the surface molecules through oxidation. Corona treatment sets up an electrical field around the film that ionizes the air molecules directly in its vicinity. The ionized air molecules, usually in the form of ozone and/or nitrous oxides, oxidize the surface of the plastic film.
Measuring and testing
Surface energy is measured in dynes. Generally speaking, the surface energy of a plastic film or part should be approximately 10 dynes higher than the surface tension of the ink or coating that will be applied. Options for testing surface energy include dyne solutions, peel tests, and contact-angle measurement.
Dyne solutions: These are made up of precise combinations of two chemicals that result in a liquid of known surface tension. How that solution reacts on the surface of a plastic film or object determines the plastic’s surface-energy level. Keep in mind that the chemicals are hazardous.
Peel test: This method relies on a machine that accurately and precisely measures the amount of force that is required to pull tape from the plastic’s surface. The test requires a relatively large, flat area, and the tape must be rubbed thoroughly after application to ensure intimate contact with the plastic.
Contact-angle measurement: This is the most accurate―and costly―method for determining surface energy. It relies on a dynamic contact-angle tester with a liquid of known surface tension (distilled water is commonly used). The process involves placing a drop of liquid on the substrate and then measuring certain angles that the beard forms with the substrate’s surface. The testing unit should measure advancing, receding, and static-contact angles for maximum precision.