Peter Swanson explains how plasma treatment can assist with difficult-to-bond substrates
Although plasma rarely occurs naturally on earth, we can still see it in our everyday lives. Lightning, the Northern Lights and the sun are examples of naturally occurring plasma. Manufacturers can use plasma treatment to alter the surface properties of substrates and increase the adherent forces they generate with adhesives and coatings.
Imagine placing a droplet of adhesive onto a horizontal surface. The more the droplet spreads to cover a greater area, the greater the wetting of the adhesive to the substrate. Wetting is a prerequisite of adhesion, so a good understanding of the factors that influence it is important for a project to be successful. The surface energy of the substrate must be greater than that of the adhesive for the adhesive to wet to the substrate.
Plastics such as polyethylene, polypropylene and polytetrafluoroethylene (PTFE) have low surface energies, typically between 20 and 40mN/m, which makes them difficult to bond. When the surface energy of a substrate is too low for an adhesive to sufficiently wet to it, plasma can be applied to alter the chemical groups on the surface, increase wettability and create bonding anchors for the adhesive.
Plasma treatment may be done in a closed chamber (low-pressure plasma), or by directing the plasma energy from a nozzle (atmospheric pressure plasma). These nozzle-type plasma treatment devices use a high voltage discharge to produce an electric arc. A gas is directed through the electric arc, which excites the gas particles and converts them into plasma. The plasma then passes through the nozzle, onto the substrate to treat it.
As well as increasing the surface energy of a substrate, which increases the wettability, plasma cleans surfaces and removes particles of dust that would otherwise impede bonding, printing and coating.
Many manufacturing sectors can benefit from the use of plasma technology on their production lines, and it is a well-established, recognised technology.
Medical device manufacturing
Adhesives are often used for the assembly of medical and life science devices including catheters, injection needles, tube sets and filters. Medical device manufacturers may utilise some specialist plastics, such as polyether ether ketone (PEEK), polyether block amide (PEBA) and styrene-acrylonitrile (SAN), for their specific properties. Some plastics used in the medical and life science industry have very low surface energies, so are difficult to bond; plasma treatment prior to adhesive bonding can be an enabling process.
Manufacturers of medical devices are subject to stringent regulations to ensure the safety of all medical staff and patients. For this reason, adhesive companies often have their medical-related products tested to either the ISO 10993 or USP Class VI standards, or a combination of both, to ensure the cured adhesives are biocompatible and suitable for consideration in these industries.
Increasingly, printed circuit boards are conformally coated to protect them from the environment. Plasma treatment can be used to activate the surface of all the circuit board parts to allow easier bonding and better adhesion of the conformal coating.
Plasma surface treatment is a “dry” process and can replace environmentally damaging chemicals and primers. Plasma can be used for the surface cleaning and removal of organic residues or oxidation from surfaces of printed circuit boards before bonding, sealing, encapsulation or potting, and can clean the surface of the circuit board, the components and the laminate to be contamination free.
When bonding two surfaces together, automotive manufacturers can reap considerable benefits from the use of plasma treatment. For example, modern headlights and rear-lights use LEDs, which are intended to last the lifetime of the vehicle. To ensure this longevity, bondlines must keep moisture out effectively. Plasma treatment can be used to ensure uniformity of the adherent force and increase the strength of adhesion, providing a waterproof joint.
Plasma is not limited to natural phenomena such as stars and the Northern Lights. Manufacturers should consider plasma treatment if they are frequently faced with difficult-to-bond substrates.
Peter Swanson is MD of Intertronics