When bonding dissimilar types of materials, surface treatment can be used to improve adhesion

Whether bonding metal to plastic, silicon to glass, polymers to other polymers of different durometers, biological content to polymeric microtiter plates or even bonding to polytetrafluoroethylene (PTFE), plasma can be used to promote adhesion.

Adhesion promotion can be achieved by increasing the surface free energy in many ways, including precision cleaning, chemically or physically modifying the surface, increasing surface area by roughening and primer coatings.

It is not a question of whether plasma is effective or not. Plasma is the king of surface activation and the best technology available. It just depends on the circumstances of the application.

Increasing surface energy

When a substrate has a high surface energy, it tends to attract. For this reason, adhesives and other liquids often spread more easily across the surface. This ‘wettability’ promotes superior adhesion using chemical adhesives.

On the other hand, substrates that have a low surface energy – such as silicone or PTFE – are difficult to adhere to other materials without first altering the surface to increase the free energy.

There are several plasma methods to increase surface energy, including physical and chemical plasmas along with Plasma Enhanced Chemical Vapor Deposition (PECVD) coating surfaces. In addition, plasma can increase the surface area of bonding by nano-roughening a surface. Surfaces that are highly ordered, or very crystalline, tend to have very low surface energies. To disrupt that order, ionised plasma gas is utilised to bombard the surface.

The most common and affordable options are helium, nitrogen and argon. The selection of the type of gas is determined by the amount of ion momentum required to disrupt the surface order.

Another method of increasing surface energy is to create a polarisable group on the surface by utilising chemical plasma. For example, O2 plasma can be used to create surface hydroxyls that allow liquids to spread through hydrogen bonding mechanisms.

Adhesion to non-stick coatings

Plasma technology can also be used to control the surface chemistry of PTFE to improve bonding, not only for adhesives, but also inks, coatings, and biomaterials.

Although ammonia gas plasma activation is traditionally used for this purpose, PVA TePla, a system engineering firm that designs plasma systems, has developed an alcohol PECVD process that improves bonding strength 1.5 times over ammonia and 8.5 times over the untreated surface.

The process also extends surface activation lifetimes. Whereas downstream processing and staging time was once confined to a six-hour window, it now extends several weeks, providing more flexibility in manufacturing environments.

This technique opens the door to new methods of chemically engineering surface properties of PTFE. The ability to selectively functionalise the surface with primary amines, hydroxyls, and carboxylic acids means that engineers can broaden the use of this material in medical technology.

Adhesion of biological molecules

Gas plasma can also provide surface conditioning of in vitro diagnostic platforms prior to the adsorption of biological molecules or biomimetic polymers. This includes precision cleaning of the substrate at the molecular level, along with raising the surface energy to improve surface assimilation of the intended content.

Microtiter and multiwell plates are often made of polystyrene, which is extremely hydrophobic. However, if you treat polystyrene with oxygen plasma it will become hydrophilic. This allows aqueous solutions containing biological content to spread and deliver biomolecules to the surface while providing a hydrogen bonding platform to adhere them.

However, some in vitro diagnostic substrates require more selective surface chemistry to immobilise a customer’s proprietary molecules. For this, PVA TePla has recently developed methods for chemically functionalising various polymer platforms for the selective adhesion promotion and conjugation of bio-active molecules.

Silicone overmoulds

Silicone overmoulding is often utilised to protect electronic boards from outdoor weather conditions. Silicone is often preferred due to its low water absorption, wide temperature range of use (typically -50 to 204°C), thermal stability, electrical resistance, and stability to ultraviolet light exposure.

Unfortunately, the topography of a PCB means the silicone must bond to many types of materials, including polymers, metals, alloys, ceramics and the FR-4 board itself, all of which have unique surface energies and chemistries.

Without proper adhesion, silicone can begin to delaminate, not only at the edges of the PCB board but also in the form of small air pockets on, or around, components. This can lead to moisture ingress and subsequent corrosion or electrical shorts.

In terms of surface energy, the best strategy is to deposit a thin film coating over everything so the silicone only has to bond to one surface energy. A process using plasma can basically harmonise all of the many surfaces and turn it into one.

To accomplish this, PVA TePla has developed a specific process starting with a precision cleaning/surface activation treatment followed by the deposition of an inert chemical primer that serves as a tie layer for the overmoulding and provides a uniform surface energy for the silicone to bond to.

Primers

Historically, chemical primers have been used to activate difficult polymer or metallic surfaces to promote adhesion. However, many of these primers are comprised of solvents – along with catalysts – that are toxic, caustic, and carcinogenic or are potential leachables.

As an alternative, PECVD can be used to deposit thin films of silicon dioxide to the substrate as an intermediate layer to improve the adhesion between a surface and a functional/linker coating or directly to a coating of choice. Within this family, the most popular are hexamethyldisiloxane (HMDSO) and tetramethyldisiloxane (TMDSO).

HMDSO, in particular, is an affordable and flexible reagent that is commercially available in a high purity, liquid form. The volatile, colourless liquid can be plasma-polymerised to create a variety of surface coatings. Depending on the composition of oxygen to HMDSO, the property of the surface can be hydrophobic or hydrophilic.

Guide wires, are a good example. To ease insertion, guide wires are often treated with proprietary surface coatings to make them more lubricious.

About the author:
Michael Barden is head of R&D at PVA TePla

Author
Michael Barden

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