How to activate the surfaces of difficult-to-bond materials

The first man-made plastic Parkesine was patented in 1856 by Alexander Parkes and many other plastics have since been developed. Today, these materials are a mainstay for many manufacturers. However, despite their favourable physical properties, some present challenges in bonding well with adhesives. Here, Peter Swanson, managing director of adhesives specialist Intertronics, explains why some materials are difficult to bond and how the challenge can be overcome.

Wetting is a prerequisite of adhesion, so a good understanding of the factors that influence it is important for a project to be successful. Imagine placing a droplet of adhesive onto a horizontal surface. The more the droplet spreads to cover a greater surface area, the greater the wetting of the adhesive to the substrate. The surface energy of the substrate must be greater than that of the adhesive for the adhesive to wet to the substrate.

Challenging materials

Plastics such as polyethylene, polypropylene and polytetrafluoroethylene (PTFE) are difficult to bond because they have low surface energies, typically between 20 and 40 dynes. Composite materials can also pose challenges for manufacturers because they comprise more than one material, each with different surface energies and wettabilities. Often, one of these materials is a polymer, which will already have a low surface energy.

Plasma surface treatment is an effective way to create bond sites on difficult-to-bond surfaces and increase the surface energy, so that adhesives can bond to these plastics more easily.

Plasma surface treatment

Plasma treatment can be performed 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 a plasma. The plasma then passes through the nozzle, onto the substrate.

The plasma causes the generation of oxygen and hydroxyl groups on the surface of the substrate, in a process called surface functionalisation. These groups act as bond sites by forming covalent bonds with molecules in adhesives, which increases adhesive strength. Plasma surface treatment can increase the oxygen percentage close to the surface of the substrate by a factor of three, resulting in shear strengths above 20 MPa.

In the case of the Relyon Plasma Pulsed Atmospheric Arc Technology nozzle-type plasma generators, the use of a unipolar pulsed high voltage source and a vortex flow in the nozzle prevents the arc from stabilising at a “hot spot”. This makes these devices particularly compact, robust and long-lasting.

As well as increasing the surface energy of a substrate, which increases the wettability, plasma is capable of cleaning surfaces and removes particles of dust that would otherwise impede bonding, printing and coating.

Intertronics supplies a range of plasma devices suited to different applications. Due to its small size and ease of manoeuvrability, Intertronics’ Relyon Piezobrush PZ2 handheld device is a good choice for pre-production or low-volume work. A variant of the Piezobrush PZ2, the PZ2-i, can be mounted to a robot for automated or semi-automated applications. Alternatively, for larger scale faster production, more powerful plasma devices are available, such as the Relyon Plasma PB3 Plasmabrush.

Applications in manufacturing

Many manufacturing sectors can benefit from the use of plasma technology on their production line. For example, adhesives are often used for the assembly of medical and life science devices including catheters, injection needles, tube sets and filters.

Medical device manufacturers often use specialist plastics, such as polyether ether ketone (PEEK), polyether block amide (PEBA) and styrene-acrylonitrile (SAN), due to their specific properties. These plastics have low surface energies, so are difficult to bond. Plasma treatment prior to adhesive bonding can be an enabling process.

The electronics manufacturing industry also has a lot to gain from plasma surface treatment. Increasingly, printed circuit boards are conformally coated in order 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.

The production of Parkesine ceased in 1868 but its legacy lives on. Alexander Parkes developed a material with properties that lent itself to many applications and now, plasma surface treatment can be used to build further on the potential of plastics in manufacturing applications.

Peter Swanson

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