Q&A: Plasma Treating
by Andy Stecher
Plasma provides plastic part suppliers a safe, cost-efficient, easy-to-operate and environmentally friendly way to clean and improve the adhesion of inks, coatings and adhesives to plastic. Yet, for many, plasma remains a bit of a mystery. Here are some of the basics.
Question: What is plasma?
Answer: Plasma is produced in items as commonplace as fluorescent light bulbs or spark plugs. Plasma, referred to as the fourth state of matter, is simply ionized gas that conducts electricity. Atmospheric plasma generators take advantage of the high temperatures found within electrical arcs. When plasma comes into contact with a plastic’s surface, its energy modifies important chemical properties and increases the surface energy. Surface energy helps determine how easy or difficult a plastic is to adhere to.
While designing plasma generators requires a good deal of expertise in surface chemistry, using plasma in production requires virtually no special training or tools at all. Atmospheric plasma is a source of atoms and radicals that are carefully selected for cleaning, surface activation or depositing thin protective coatings. For example, adding oxygen to plasma is used to cure polymer films, whereas nitrogen-enriched plasma combined with silanes is used to deposit a silicon nitride plasma coating. Atmospheric plasma pretreatment is one of the most efficient processes for cleaning, activating or coating plastics and composites.
High-quality plasma equipment is no more complicated to install and use than most plastic decorating equipment. In many cases, the plasma system requires only a source of electricity and compressed air. Plasma may be integrated into an existing line by installing fixed plasma jets for treating simple shapes or by moving the plasma heads for more complex three-dimensional parts.
Question: Is atmospheric plasma treatment safe?
Answer: Atmospheric plasma treatment is completely safe and environmentally friendly. Plasma treatment generates a small amount of ozone, so the equipment should be operated in a well-ventilated area. The plasma source does not present an electrical shock hazard, and the plasma flume is mildly warm to the touch. Of course, standard safety precautions should be used, as with any industrial equipment that uses electricity and compressed gas.
Plasma provides many health and safety advantages when compared with other ways of pretreating parts. There are no dangerous chemicals, solvents or hazardous air pollutants. Plasma also does not have the exposed flame or combustible gases common to flame treatment.
Question: Will plasma clean dirty plastic parts?
Answer: Atmospheric plasma makes it possible to clean and process parts inline. Energetic plasma can selectively break apart organic substances, such as mold release agents, additives, softeners, plasticizers, hydrocarbons and other soils frequently found on the plastic surface. Using ultrafine cleaning, contaminants can completely be removed from delicate or sensitive surfaces. Plasma cleaning also removes even the smallest dust particles from plastics, which adhere tightly to the surface because of additives. Plasma causes these particles to detach completely from the substrate.
The effectiveness of plasma for industrial cleaning should be tested using the same soils and contaminants that will be present in production. Generally, plasma is ideal for removing thin layers of soils that are very difficult to remove. Plasma provides 100-percent cleaning, has no dilution effect, contains no additional consumables, cleans deep into pores and does not take up any additional space since cleaning and activation can take place in one step.
Question: How does plasma improve adhesion to plastic?
Answer: When coatings, inks or adhesives need to be applied in subsequent steps, surface activation is indispensable. For example, polymers often require surface modification so they can be printed with water-based inks, bonded with solvent-free adhesives or coated with highly crosslinked UV-curable materials.
The highly charged electrons, radicals and ions present in plasma react with chemically inert species on the surface of plastics to enhance the surface energy of the material. Atmospheric plasma is particularly effective for nonpolar materials, such as plastics with long-chain polymers. Such nonpolar surfaces are notoriously difficult to bond, coat or decorate. Because the ions and radicals in plasma modify the plastic’s surface tension, these difficult materials can be processed more easily, providing greater freedom of substrate choice for designers and greater process reliability for engineers.
Plasma also provides greater consistency for manufacturers that use recycled plastic and composite materials. The result is better, and more repeatable, wetting capability of the surfaces.
Plasma is effective prior to printing since plasma conditioning makes adhesion of solvent-free inks possible on difficult-to-print surfaces, such as some varieties of polypropylene, polyethylene, polyamide and polycarbonate.Question: How long does the effect of plasma last?
Answer: The beneficial effect of plasma treatment can last for several minutes, hours, weeks or even months. Unfortunately, the answer depends on a number of specific factors, such as the plastic substrate being treated and chemical composition of the plasma. Work with the plasma equipment supplier to test the effect of plasma in their lab before implementing the process in a production setting. However, even when the effect of plasma is relatively short-lived, plasma treatment typically lasts longer than other methods of cleaning or activation, such as solvent wiping for plastic parts.
Question: How can I develop a process, and how do I control and monitor it?
Answer: Good question. It’s one thing to have plasma work in theory; it is another to design a process that works in the plant. For many applications, the cost of failure is high, and it is important to monitor the process and get feedback into a production monitoring system. Good planning and investing in the right tools helps ensure good quality, reduces scrap and rework and prevents costly failures.
Any important aspect of a new manufacturing process should be proven in the lab and under pilot test conditions before launching full-scale production. Plasma needs to be integrated into the overall manufacturing process, including application, curing, material handling and other upstream and downstream tasks. A reputable plasma equipment supplier should help develop and optimize a plasma surface conditioning process in their laboratory and then conduct a field trial to establish and document a production process.
To ensure uniform quality, the plasma stream should be spectrally monitored on a continuous basis using optical sensors built into the plasma nozzle. The plasma amplitude, in the relevant spectral range, also is measured. Taken together, these measurements characterize the plasma. Along with plasma intensity and the distance to the substrate, the travel and rotational speed of the plasma nozzles are important factors that affect surface treatment quality. A motion control system is available to ensure that these critical parameters are monitored and maintained. Finally, proper control of the supply air and other additives ensures reproducible process parameters, such as temperature and intensity. Plasma providers can supply monitoring units, including flow monitoring.