Contaminants such as oxides, water, organic substances and dust often cover a surface in layers because of their natural presence in air.
Although the surface may appear to be clean to the human eye, such contaminants can impair further treatment such as bonding, printing, painting or coating. Here Peter Swanson, Managing Director of adhesives specialist, Intertronics, discusses plasma cleaning technology and its benefits.
As well as exposure to air, technical processes can leave a surface tainted with oils, release agents, compounding ingredients, monomers and exuded low molecular weight species. This fine soiling of both organic, inorganic, dust and microbial contaminants will typically have low wettability, leading to incomplete surface covering when applying an adhesive, which can further reduce bonding strength.
Generating plasma
To improve wettability and increase bonding strength, engineers may want to consider trialling plasma cleaning. During the process, plasma — partially ionised gas — initiates a multitude of physical and chemical processes that treat the surface, removing contaminants without the use of additional chemicals.
The electrons and ions in plasma can accelerate to very high energies and collide with gas molecules to produce short-lived, chemically active species, such as atomic hydrogen, nitrogen and oxygen species, hydroxyl radicals, nitric oxide radicals as well as ozone, nitrous and nitric acids. These species can disinfect, clean, modify and functionalise a range of surfaces to prepare them for adhesive bonding, varnishing or printing.
Electric arcs, dielectric barrier, corona and piezoelectric direct discharges can ionise gases to create plasma at atmospheric pressure. During the process, a small fraction of gas molecules is turned into energetic electrons and ions, whereas the rest remain neutral and cold. The temperature reaches only 50 °C during piezoelectric direct discharge and 250–450 °C in the case of arc discharge.
Plasma cleaning
Plasma breaks the organic bonds of heavy organic molecules to produce lighter and more volatile ones, which evaporate from the surface and leave it in an ultra-clean state. The reactive chemical species oxidise organic contaminants to form carbon oxides and water vapour. These processes also sterilise the surface, destroying microbial contamination.
When in direct contact with the substrate, electric discharges will erode it at the micrometre scale, creating microstructures that improve the mechanical binding of an adhesive. Plasma can also deposit polar hydroxyl and nitric oxide groups on the cleaned surface to increase its energy and wettability. The result is that an adhesive will wet more efficiently and can fill microstructures as a result of capillary action.
The benefits of cold plasma
The cold plasma method avoids wet chemistry and requires no expensive vacuum chambers, improving the cost, safety and environmental impact of the process. It also offers fast processing speeds, which can be beneficial in some applications. In addition to the energetic ions and electrons, the surface is irradiated by very short-wave ultraviolet light, enhancing the cleaning effect.
Cold atmospheric pressure plasma is very effective, does not overheat the surface and does not visually affect it. It requires no toxic chemicals and the reactive chemical species decay rapidly after treatment, making it safe and environmentally friendly. Compared with low-pressure plasmas, atmospheric plasma equipment is usually less expensive.
Our customers have had success using plasma cleaning for one- and two-dimensional products, cylindrical products, complex structures and granulates and powders.
This has occurred in a range of sectors, including medical, electronics, food and cosmetics, for example. Atmospheric plasma cleaning technology is available in compact and easy-to-integrate forms. For example, the Plasmabrush PB3 is a compact plasma generator that can be easily integrated into automated treatment systems. For smaller objects or laboratory work, the low-power, handheld Piezobrush PZ2 may be suitable.