Use our plasma system for adhesion improvement

When plasma is used to functionalize a surface prior to gluing, the resulting bonds will display a strong adhesion improvement. Plasma can be used on a wide variety of materials: metals, glass, ceramics, and even natural materials such as woods and textiles respond very well to plasma functionalization. For several our customers, it is especially important that many polymers with typically “non-stick” surfaces can be successfully glued and bonded following plasma treatment.

This surface functionalization achieved by plasma processing relies on a combination of effects: ultra-fine surface cleaning from organic contaminants, modification of the surface topography, modification of the crystalline structure of the surface layer, and deposition of functional chemical groups. Metals can be cleaned of hard oxides, restoring the sheer metal surface.

The atmospheric pressure of our plasma systems is generated using air or typical industrial gases such as hydrogen, nitrogen or oxygen. For our customers, this means a huge improvement regarding cost-efficiency, process safety and environmental compatibility, as expensive vacuuming equipment or wet chemistry are simply no longer necessary. Fast processing speeds make our system suitable for a wide variety of industrial applications.

Plasma pre-treatment optimizes the quality of your adhesive bonding

The following effects of plasma pre-treatment provide ideal conditions for strong, high-quality adhesive bonds even between dissimilar materials:

• Ultra-clean surface. A surface which is to be bonded must be free from contaminants. Even visually clean surfaces can contain contaminants such as adsorbed organics, water, monomers, release agents, or oils. Ultra-fine clean state is difficult to achieve with Conventional cleaning techniques often leave residues, whereas plasma treatment achieves the desired ultra-fine clean state which is crucial for achieving an optimum bond.
• Oxide-free metal surface. The best adhesion to metal surfaces is achieved when they are free from metal oxides. However, when exposed to air metals oxidize almost instantly. Thus, the time between the removal of the oxide layer and the application of the adhesive usually needs to be minimized down to milliseconds. Integrating our plasma system into your process allows you to do just that.
• Strong surface. In the case of plastic polymers, which are formed by molding and extrusion processes, the uppermost nm-width surface layer consists of polymer molecules of low molecular weight that are not cross-linked. This surface layer is mechanically weak. Removing this weak layer and cross-linking the polymer molecules in the remaining layers improves adhesion strength considerably.
• Wettable surface. For the adhesive to cover (wet) the surface efficiently, the surface energy of the adhesive should be lower than that of the substrate. However, mechanically strong adhesives and paints usually have high surface energy. This poses a serious problem for their use with most plastic polymers, which typically have very low surface energy. Plasma treatment solves this problem by increasing substrate surface energy and therefore wettability significantly, even with polymers.
• Amorphous surface with diffusion of polymers. Plastic polymers have a semi-crystalline structure with regions of either crystalline or amorphous materials. Diffusive bonding, achieved by glues, is stronger with the amorphous material, when the polymers are free to diffuse into the glue. Plasma treatment promotes the scission of polymer molecules present on the surface, which facilitates their diffusion and strengthens their bond with the glue.
• Chemically functional surface. On the molecular level, the adhesion between two surfaces is caused by either electric attraction between the surface molecules and the molecules of the adhesive, or by their chemical bonds. The former type, called dispersive adhesion, is strong when polar molecules cover the surface. While surfaces of plastic polymers are typically non-polar, the formation of a layer of polar molecules functionalizes the surface. The latter type, called chemical adhesion, forms the strongest joints. However, chemical bonding between dissimilar materials is not possible. An intermediate layer of molecules having chemical affinity to both, the surface and the adhesive, functionalizes the surface enabling very strong chemical bonding. Plasma treatment facilitates both dispersive adhesion and chemical bonding.
• Microscopically rough surface. When a surface is microscopically rough, it is wettable, i. e. capillary action will cause the surface pores and irregularities to fill with adhesive as soon as it is applied. This increases the mechanical strength of the adhesive bonding. It is well known that plasma treatment improves the wetting properties of almost all materials.

Cold atmospheric plasmas

Plasma is a partially ionized gas. Electric arcs, the dielectric barrier, as well as corona and piezoelectric direct discharges ionize gases at atmospheric pressure creating plasmas. The charged particles – electrons and ions – accelerate to very high energies. Only a small fraction of the gas molecules is turned into energetic electrons and ions; the rest of the gas remains neutral and cold. In the case of piezoelectric direct discharge, its temperature reaches only 50°C. When there is arc discharge, the arc volume reaches temperatures of 6.000 – 12.000°C. However, after leaving the discharge volume, the gas quickly cools down to 250-45°C. When processing speeds are set accordingly, these temperatures do not damage the surfaces treated. While the plasma remains cold, the very energetic electrons and ions collide with the gas molecules, producing large quantities of short-lived chemical species, such as atomic H, N and O species, OH and ON radicals, ozone, nitrous and nitric acids, as well as various other molecules in metastable excited states. They make this type of plasma chemically very active.

Adhesion improvement through plasma treatment

Upon contact with the treated surface, the chemically active cold atmospheric plasma initiates a multitude of physical and chemical processes. The main reaction agents are highly reactive short-lived neutral chemical species, which are produced by the plasma in large quantities. Additionally, when the electric discharge touches the treated surface, the latter is also irradiated by VUV light and bombarded by the energetic electrons and ions. Although the quantities of the charged particles are small, their highly reactive nature strongly enhances the effects of the plasma. The following processes contribute to the promotion of the adhesion by the plasma treatment:

• Plasma cleans the surface. Plasma breaks organic bonds of heavy organic molecules producing lighter and more volatile molecules evaporating from the surface. Further, reactive chemical species oxidize organic contaminants forming carbon oxides and water vapor. As the plasma breaks contaminants turning them into vapor, no residues are left on the surface, leaving the latter in an ultra-fine clean state.
• Plasma reduces metal oxides. Plasma discharges, ignited in the forming gas, which typically contains 5 % of hydrogen and 95 % of nitrogen, produce large quantities of reactive hydrogen species. By contact with oxidized metal surfaces, they react with metal oxides reducing them to metal atoms and water vapor.
• Plasma strengthens the surface. With a high treatment strength, plasma removes nm-scale weak surface layers having the lowest molecular weight. The bonds of the polymers, broken by the plasma, cross-link forming a stronger surface layer.
• Plasma induces a phase transitions and polymer molecules scission. With a further increased treatment strength, plasma induces a phase transition from the crystalline to the amorphous structure of the surface layer. The latter diffuses easily into the glue, increasing the strength of the diffusion adhesion. Moreover, plasma breaks polymer molecules, exposing their scissed open ends on the treated surface. This further increases the diffusive bonding strength.
• Plasma deposits chemically functional groups and increases the surface wettability. By reacting with the polymer molecules, plasma species deposit polar OH and ON groups on the cleaned surface significantly increasing the energy of the surface and its wettability. As the results, the subsequently applied adhesives wet the surface efficiently and fill the microstructures due to the capillary action. Moreover, by adding specific chemical substances, plasma can deposit specialized functional groups or even polymerize the surface to enable the strongest chemical adhesion.
• Plasma microscopically roughens surface. Electric discharges having direct contact with the substrate, especially the electric arcs burning on the metal substrate, when the latter is used as a cathode, erode the substrate surface on the micrometer scale. This creates microstructures that are filled by the adhesives improving their mechanical binding to the substrate.

Advantages of plasma processing

Plasma cleans, strengthens and chemically functionalizes the surface. All these effects, which are required for improved adhesive bonding, are achieved simultaneously in a single step. Most importantly, the plasma processing works at atmospheric pressure. Its advantages comparing to standard chemical and vacuum plasma cleaning processes include:

• Ultra-fine cleaning, no residues
• Gentle, non-destructive surface treatment
• No wet chemistry
• Air or cheap non-toxic working gases
• Environmental friendliness
• No expensive vacuum equipment
• Fast processing speeds
• Easy integration into existing production lines

Adhesion improvement with tools of relyon plasma

To cover a wide spectrum of industrial, medical and laboratory applications, relyon plasma GmbH developed a series of plasma processing products designed to improve adhesive bonding, such as printing, coating, painting and gluing:

Plasmabrush® PB3

• plasmabrush® PB3 is a universal highly reliable plasma generator based on our proprietary Pulsed Atmospheric Arc (PAA) Technology. With its power of 1 kW, very compact dimensions and exceptional long-time stability, this generator is well suited for integration into high-speed industrial production lines.

Plasmacell P300

• plasmacell P300 is a complete “turn-key” plasma treatment solution. It includes all components required to establish an effective plasma processing conforming to industry standards and regulations: Plasmabrush PB3 mounted on a programmable high-speed X-Y-Z positioning system, compressed air supply and exhaust filtration systems. Enclosures with electronics and the treatment chamber, together with an electronic display, create a clean, safe and efficient working environment, ready for operation.

Plasmatool

• plasmatool is a handheld instrument based on plasmabrush PB3, ergonomically optimized for safe manual operation. Together with a portable module including a high voltage supply, an air compressor and controlling electronics, it enables highly efficient plasma processing of large structures, areas that are difficult to access, or where automation is not possible or practical.

Piezobrush® PZ2

• piezobrush® PZ2 is a low power handheld plasma generator, which is based on our proprietary Piezoelectric Direct Discharge (PDD) Technology. It enables manual plasma processing work in laboratories. It can create corona and dielectric barrier discharges and apply them for precision ultrafine plasma cleaning and chemical functionalization of small components.