Future Trends in Corona Treatment of Narrow Webs

By Alexander Rau, Product Manager at Ahlbrandt, a Baldwin Technology

In recent years, the world has witnessed a growing awareness of environmental concerns, prompting many to reduce their ecological footprint. One such industry is flexible packaging, where a significant transformation is underway towards more sustainable practices, reduced energy consumption, the introduction of recycled plastics and minimized emissions for the environment. 

These transformations are joined by new and adapted printing processes that require a more precise and sometimes stronger corona treatment. Narrow web and label printing often lead in these new technologies and will therefore be the first to require a premium corona treatment.    For these applications, Baldwin Technology released the newly designed Corona treater SlimLine together with a suitable clean air purification system CAP.  

Energy Saving  

In the digital space, label and narrow web flexo printing and UV LED curing is becoming popular as it is fast and efficient, consuming less energy than mercury UV lamps or the hot air drying of conventional inks. Beside the benefits of reduced energy consumption, the UV LED process emits no volatiles or ozone, reducing the environmental impact and improving safety measures for operators. Although UV LED inks contain no solvents, a strong adhesion of the print requires a relatively high surface tension for plastic substrates, or in addition, a precoating of the substrate with primers. Higher surface tension can be achieved by a proper corona treatment that introduces polar groups on the substrate surface, leading to improved wetting and adhesion. The dyne level should be at least above 44 mN/m to achieve strong adhesion and printing results. For optimal adhesion, UV-ink suppliers recommend optimal dyne levels for their formulations or special primer types.  As different substrates like PE/PP, PET, coated paper or metallized substrates require very different treatment intensities to reach the required dyne level, the power must be adjusted accordingly. Beside the power setting, a precise adjustment of the electrodes that form the discharge gap is important. Some substrates, like metalized films, can be very sensitive to the setting of the discharge gap. Even small changes of the gap by 0,3 mm can lead to a change of 4 mN/m in the resulting surface tension. Therefore, it is crucial to have adjustable electrodes allowing a precise setting of the discharge gap even in small narrow web applications. 

  While in some cases the corona treatment can substitute a primer coating before UV printing, there are film and ink types that require a primer coating as even high levels of surface tension do not lead to the required adhesion level without a primer. But even in this case, a corona treatment is preferred, as best adhesion is achieved with a corona treatment before the primer coating step. 

Use of Recycled Plastics 

The environmental impact of plastic waste has spurred a global call for action. Brands and consumers increasingly demand sustainable packaging, and the use of recycled plastics offers a way to a second life or more for materials. Collecting and reusing processes is growing in popularity. Cleaner and correctly sorted recycled plastics are becoming more available. However, the process is accompanied by some challenges, including degradation due to additional processing steps and less homogenous material composition compared to virgin material. Handicaps like this can be compensated by additives improving the properties of the recycled material or by simply mixing a certain percentage with virgin material. 

The higher variation of material composition also impacts surface properties that are highly influenced by the migration of low molecular chains like slip agents. To circumvent variations, a higher intensity of corona treatment must be applied to compensate for fluctuating film quality.  This means that, compared to virgin material, the maximum power of the corona unit must be increased. When investing in new lines, it is best to plan for future changes in material as equipment options are reviewed. However, even existing lines can be easily upgraded to handle today's wider variety of materials. 

Exhaust Air Cleaning 

Corona treaters produce ozone as a byproduct, a potentially harmful gas that needs to be effectively managed and removed from a working environment. This is done by guiding ambient air through the discharge area to an exhaust system. The location of the exhaust outlet is critical, and should be remain within ozone limits. Ozone removal techniques can be implemented but should meet local legislation standards which can rapidly change.  

Among the different removal techniques, catalytic converting is the most economic and safest solution. It works simply by guiding the exhaust air through a catalytic bed of  manganese-/iron-oxide, whereby ozone (O3) is converted to oxygen (O2) without the need for additional heating of the catalytic bed. The reaction produces a slight heating which allows the converter to be positioned inside or outside of the facility. For longer lifetimes, the catalytic bed can be protected by prefilters that remove humidity and catalyst poisons. Unlike absorption converters that are working with activated carbon, catalytic converters have a non-flammable bed without the risk of spontaneous ignition.   

Summary  UV-LED printing is fast growing in narrow web and label printing. For printing on flexible films, a powerful corona treatment and/or primer coating is required. A corona treatment is set before the primer coating for best results. For some ink/substrate combinations, the primer coating can be substituted by a pure corona treatment. 

Given rising consumer concerns and a rapidly shifting regulatory environment, the inclusion of recycled material in flexible packaging will most likely rapidly increase. 

Both UV-LED printing and recycling substrates require corona treatment with high intensity and a precise treatment setting. Baldwin’s Corona SlimLine is designed for comfortable operation and  tackles these challenges with enough power reserve. With its robust design, it provides easy setting of the discharge gap while reducing the accumulation of dirt accompanied. 

Ozone removal from exhaust air is beneficial for the larger environment and avoids exceeding any ozone limits surrounding the exhaust area. The preferred technology is the catalytic ozone conversion which guides the exhaust air through a catalytic bed. Based on this technology, Baldwin developed the CAP 040 air purification system for narrow web applications. For long-lasting catalyst function, it is equipped with a prefilter and its small design allows for it to be easily integrated in every exhaust pipe system.