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.
To improve wettability and increase bonding strength, engineers may want to consider trialling plasma cleaning. During the process, plasma — partially ionized 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 ionize gases to create plasma at atmospheric pressure. During the process, a small fraction of gas molecules are turned into energetic electrons and ions, while the rest remain neutral and cold. The temperature reaches only 50 ⁰C in the piezoelectric direct discharge and 250-450 ⁰C in the case of arc discharge.