Commonly-used plastic materials such as polypropylene have a very low surface energy, which essentially makes them non-stick in nature.
The surface energy of a material determines how well it will bond to a liquid, whether that be an adhesive, ink, paint or other liquid coatings. In order to achieve a good bond, the surface energy of the material must be greater than the surface tension of the adhesive, ink, paint or other liquid being applied. When this is achieved, the resultant molecular attraction will draw the liquid down towards the surface of the material, encouraging the liquid to flow and ‘wet out’ the surface, maximising contact and adhesion between the surface of the material and the liquid. Conversely, if the surface energy of the material is lower than the surface tension of the liquid, the molecular attraction will be weaker, resulting in the repelling of the liquid and causing it to bead up rather than flow and ‘wet out’ the surface.
Plasma and corona surface treatments are highly efficient and reliable methods of activating or pre-treating plastics such as polypropylene, modifying the surface chemistry and raising the surface energy to prepare the material for further processing. Both treatments bombard the surface of the material with a highly energised, ionised gas which breaks up and forms new molecular bonds on the surface of the material. This process both increases the surface energy, which in turn increases liquid wettability and leaves behind new chemically-reactive bonding sites.
It is important to note that all of this is happening at a molecular level. Plasma and corona treatments do not alter the bulk properties of the material. And although the change is not visible to the naked eye, the newly treated surface is now optimised for further processing.
As well as activating and increasing the surface energy of plastic materials, plasma and corona surface treatment also cleans the surface of the material at a microscopic level, removing organic contaminants such as oils that could inhibit bonding. This eliminates the requirement to use harsh chemical cleaners which could discolour or etch the surface of the material and is suitable for even the most sensitive of materials.
MAKING A CHOICE
Plasma or corona: both terms are often used interchangeably, but are created in different ways. Plasma is matter that exists in the form of ions and electrons. Basically, it’s an ionized gas: a gas (typically just atmospheric air) that has been electrically charged with freely moving electrons in both the negative and positive state. Corona electrodes ionize the air around them (which is a gas) so therefore the resultant corona discharge is actually a plasma, so the end effect is the same.
One important difference is that the plasma discharge is potential-free, so you could use it for treating sensitive electronic components, for example. On the other hand, corona discharge is high-voltage, so unsuitable for electronic components.
The choice between them is typically down the material type and the size/shape of the component.
Tantec offers two plasma technologies: atmospheric plasma and vacuum plasma. Atmospheric plasma uses a nozzle-type device for localized treatment at high speed, and is typically line- or robot-mounted. It is good for treating specific areas, for example a small bonding area on a large component. Typical applications include high-speed extrusion lines, electronic components, automotive components and robot mounted applications.
Vacuum plasma provides a comprehensive, all-over treatment of the component, even if it is complex in shape. The plasma treatment is performed in a chamber at very low pressure which allows the plasma discharge to flow in around the component. Typical applications include medical device components, delicate electronic components and automotive components that require a uniform, all over treatment.
Corona, on the other hand, is typically associated with the treatment of continuous web plastic films: food packaging, label materials and the like before printing, coating or laminating. However, corona lends itself to a wide variety of formats including systems for treating large flat sheets or boards and systems dedicated to the treatment of medical syringes or optical lenses. Corona is a versatile technology, and Tantec is able to offer fully customised solutions tailored to specific needs and specifications.
This article was first published on the Tantec UK website: https://www.tantec-uk.com/enhancing-the-bonding-of...
