The most significant design criterion involved in corona treatment equipment selection is sizing the system to meet the specific application requirements. Power supply size in kW is determined by the treatment power required by the most difficult-to-treat substrate to be coated. Treatment power is measured in Watt Density (Watts/Square Foot/Minute), which takes into consideration not only power level, but also the length of time the power is applied. If a given material is treated at a given watt density, its surface energy will be increased a given amount. This is the basic goal of surface treatment. However, both the ultimate surface energy achieved and the amount of increase are dependent on the material’s starting surface energy. For example, applying a watt density of 1.0 Watts/Square Foot/Minute on PET at 41 dynes may raise it to 46 dynes, but applying that same watt density of 1.0 to PET at 44 dynes may raise it to only 48 dynes. Although the final dyne level is higher in the second instance, the incremental increase is less because of the starting point dyne level.
As you might expect, different materials react differently to corona treating. Some materials, such as most polyesters, accept treatment readily and will exhibit a rapid increase in surface energy under relatively low watt density levels, say 0.5 to 1.0. Other materials, such as polyethylene with some additives, accept treatment less readily but will exhibit a significant increase in surface energy under moderate watt densities, say 1.4 to 1.8. Finally, materials such as polypropylene with slip additive, which are difficult to treat, may exhibit only moderate increases in surface energy under relatively high watt densities, say 2.0 to 2.5.
Treatment levels, i.e., watt density levels, vary considerably from application to application. The materials and watt densities selected for discussion here were chosen because they are somewhat typical of extrusion coating applications. However, treatment levels can also vary significantly based on additive load and other substrate characteristics.