The success of adhesive bonding and sealing processes depends largely on the surface properties of the substrate. For highly adhesive, media-tight bonds, appropriate pretreatment is usually essential. Plasmatreat's atmospheric plasma technology requires virtually no chemicals, reduces effort, cycle times and costs, and makes long-lasting adhesive bonds possible.
As the Austrian physicist and Nobel Prize winner Wolfgang Pauli once said, “God made the bulk; the surface was invented by the devil.” Still, you don't have to make a deal with the devil to master surface properties. On the contrary, it is the upwards glance towards the sky that provides the answers. That's because one of the most efficient surface problem solvers can be found there in the form of polar light, thunderbolts, stars, and galaxies: plasma. Its highly excited state makes it suitable for modifying a wide variety of surfaces – from plastic and metal to glass, stainless steel and ceramics. Even temperature-sensitive surfaces can be cleaned, activated and coated reliably.
A question of surface energy
Industrial applications often involve permanently bonding different materials and components and protecting them from environmental influences. This can be achieved by functionalizing the surface. Functionalization involves physically and chemically modifying the surface so that the desired properties are obtained. The decisive factors involved here include both the materials and components themselves and the materials from which substrates and adhesives and sealing compounds are made. Whether the desired adhesion is achieved is often a question of the surface energy of the surface. In principle, the surface energy [J/m2] or the measured surface tension [mN/m] of the material to be wetted should be higher than that of the wetting medium. Considered more precisely, the surface energy is composed of disperse and polar components. Ideally, the phase boundary of two materials should have equal proportions of disperse and polar components. This ensures optimum surface wetting.
Is primer still the tool of choice?
While materials such as metal and glass already have a high surface energy by nature, non-polar plastics are difficult to wet because of their low surface energy. Recycling processes can also have an impact on the wettability of plastics, as treatment steps can result in chemical degradation processes or impurities. As a result, the recycled plastic then exhibits altered material properties compared to the raw product.
If the surface tension of the material to be wetted is not high enough, bonding agents are usually used. Yet this practice is associated with significant disadvantages. Primer applications always entail an additional work step in addition to cleaning and applying the adhesive or sealing compound. Moreover, solvent-based primers pose a challenge in terms of occupational safety and storage. In addition, the reclassification of raw materials (e.g., REACH) often leads to stricter labeling regulations and thus to higher expenses in handling primers.
Even if organic solvents are replaced by water in the bonding agents, this is not an optimal solution. For example, aqueous primers have significant disadvantages in the process due to longer drying times and poorer wetting properties.
Atmospheric-pressure plasma: An efficient and environmentally friendly alternative
Although primers are still the tool of choice for creating long-term stable bonds for challenging material combinations, companies are increasingly abandoning these conventional bonding agents for reasons of productivity, cost, and the environment. One efficient, reliable, and cost-effective alternative is treatment with atmospheric-pressure plasma (AP plasma). Pretreatment with Openair-Plasma increases the adhesion and wettability of surfaces, thus enabling the use of more cost-effective material alternatives and new types of material composites while maintaining the quality of the end products – a decisive competitive advantage.
The plasma process is dry, non-contact, selective in location, and very fast. In just one work step, the surface is cleaned with microfine precision and simultaneously activated at the molecular level. The technique is fully robot-compatible and inline-capable. Production speeds are often increased many times over, while the use of manpower is reduced and the error rate is lowered. This has a positive effect on costs. Whether it's structural adhesive bonding in automobiles, sealing in electronics or fast, bubble-free wet labeling with high initial strength – thanks to Openair-Plasma and modern solvent-free adhesives, manufacturing processes can now be carried out completely without chemical waste. This is equally true for printing and painting. The high surface energy generated by Openair-Plasma ensures uniform paint distribution and optimal bonding.
Modify material properties in a targeted way
For higher or more specific requirements, the surfaces of the materials are coated using the PlasmaPlus process. Among other things, these nanocoatings include corrosion barriers and can replace toxic CrVI primers and adhesive and sealing layers that promote direct bonding of disparate materials without primers and adhesives. In this process, a coating material – also known as a precursor – is introduced directly into the Openair-Plasma jet. The PlasmaPlus process makes inline processes possible at a speed that would never be achievable with conventional bonding agent processes, let alone with such a minimal use of chemicals. There are two key advantages: Consumption is generally limited to a few grams per hour and per plasma jet. Moreover, further processing steps such as painting, bonding, and injection molding can be carried out directly after the plasma treatment. Waiting times for the bonding agent to flash off or dry are eliminated.
Every surface is a plasma surface
No matter what the material basis of the substrates is: To create a bond to adhesives and sealing compounds by means of plasma, only the interface is modified by the substrate. As opposed to primers, this can be controlled with pinpoint accuracy and intensity. This means that, simply by selecting the parameters, a wide variety of materials can be pretreated on ONE system: from metals, for which high plasma intensities with high energies can be used, to temperature-sensitive plastics, which require less energy. In addition, the PlasmaPlus process allows suitable chemical bonding agents to be selected for the respective substrates. The same applies to all plasma treatments: The experience gained in the functionalization of plastics, metals, or glass can be quickly applied and exploited for a wide variety of applications. Likewise, previously known functionalizations such as hydrophobicity, hydrophilicity, or adhesion can be adapted to new materials from existing applications. This is what makes atmospheric-pressure plasma so efficient. The following examples illustrate the wide range of plasma applications and processes.
Electronics manufacturing: Improved bonding and sealing with plasma
The growing segment of e-mobility and the associated integration of touch displays, safety and sensor systems, and cameras are significantly increasing the number of adhesive and seal applications in the electronics industry. To ensure that the sensitive high-tech electronics function reliably, they need to be protected from harmful environmental influences such as moisture. Environmental and EMC seals are the tools of choice for this. Treatment with Openair-Plasma ensures a stable bond between the metal or plastic housing and the seal.
Plasma technology also offers notable advantages in the pure bonding of displays and HMIs (human-machine interfaces) by means of optical bonding. Any remaining impurities or those created during the production process are completely removed. At the same time, a process of activation takes place in which free radicals are bound to the surface of the material. This prevents what is known as the bubble effect since the increased surface tension distributes the transparent liquid adhesive perfectly over the glass surface. Bonding without air bubbles is guaranteed. Given the increased integration of touch monitors – from the automotive and railroad industries to medical technology, mechanical engineering, and aerospace – the use of plasma in optical bonding will increase even more in the future.
Another important field of application for Openair-Plasma technology in electronics manufacturing is conformal coating, the full-surface sealing of individual components or even complete assemblies. Yet the process is not entirely without its problems, as it has many variables that can have a negative impact on the final quality of the coating. These include the formation of bubbles or orange skin, uneven coatings, and delamination. Pretreatment with plasma ensures very finely cleaned, highly active surfaces, thus enabling a stable bond between the surface and the coating material.
InMould-Plasma: The solution for hard-soft composites in 2K injection molding
To satisfy industry's increasing functionality requirements, technical plastic components are often made from several materials. Hard-soft composites are particularly in demand – in tool handles or covers for car interiors, for example. If the plastic combinations are produced by multi-component injection molding, the processed materials must be thermally compatible to create an adhesive bond. This limits the choice of materials, and leads to compromises with regard to the property profile of the soft component.
Surface activation with atmospheric-pressure plasma, a process long established in plastics processing, has already expanded the range of materials so that even previously incompatible materials can adhere to each other with long-term stability. The new InMould-Plasma process goes one step further by integrating plasma treatment directly into the injection mold. Elaborate, error-prone assembling and logistics processes are eliminated. Just how the process works in practice can be illustrated by the manufacture of a lid (PP) with a sealing function (TPU) in a 3-station rotary table mold. The thermoplastic lid is injected in the first station. The lid stays on the rotary table after the mold is opened and swivels into the second station. A plasma jet is installed there. When the mold is closed, the cavity on the jet side presses on the lid and seals a channel above the surface on which later the soft component is sprayed. The plasma jet is ignited and the plasma flows through the channel, activating only the relevant area of the lid. Nitrogen is used as the process gas, thus ensuring a highly effective activation of the plastic surface. The process takes only a few seconds and does not usually affect the cycle time. Finally, the elastomer component is sprayed on in the third station. Each time the mold is opened, a finished 2-component part is demolded.
Without pretreatment with plasma, PP-TPU composites cannot develop an industry-relevant adhesive strength that persists even after the components have aged. Other thermoplastic elastomers (including TPE-S and TPE-V) have also been bonded to engineering thermoplastics such as PC, PBT, and PBT/ASA. Apart from new material combinations, the plastics processing industry benefits from the shorter production times, reduced manufacturing costs, and full process control of InMould-Plasma technology.
Highly adhesive, media-tight plastic-metal hybrid components
Electrical connectors have to function absolutely reliably, because failures can have serious consequences. For example, one car manufacturer had to recall over 8,000 SUVs because a defective wire harness connector created excessive electrical resistance. Other connector failures have led to major power outages and plane crashes. The last Mir modular space station also nearly failed because of a faulty connector. The most common reasons for technical defects are mechanical failure and corrosion of the metal contacts. The metal-plastic interface between the metal contact pins and the injection-molded housing is especially susceptible to the penetration of moisture.
With Plasma-SealTight, a plasma sealing process has been developed that enables highly adhesive, media-tight plastic-metal composites in the injection molding process, thus helping to prevent electrical connector failures. After cleaning and activating the metal contacts, this PlasmaPlus technology is used to apply a nanocoating as a bonding agent. This Plasma-SealTight layer ensures adhesion of the polymer and prevents corrosion of the contacts through the infiltration of moisture.
In another project, the prototype of a multi-pin connector was developed jointly with other companies. The result demonstrate how plasma increases the quality and reliability of electrical connectors. The surface treatment produces an assembly with high integrity and reliability with respect to moisture penetration. It withstands water pressures of up to 5 bar. The mechanical bond strength between metal and plastic is also impressive. While conventional processes achieve maximum tensile shear strengths of 20 MPa, Plasma-SealTight reaches over 50 MPa.
Long-term stable bonding in the adhesive joint
PT-Bond is another PlasmaPlus application specifically designed for bonding and sealing technologies. An organosilicon compound is added to the plasma as a precursor, while the chemical composition varies depending on the material and the application. PT-Bond coatings increase bonding forces and significantly improve sealing properties, even for hybrid material combinations with different thermal expansion coefficients.
The challenge for a well-known clothes (tumble) dryer manufacturer was to create a material combination of glass and PP plastic for the door assembly. A 1K PU foam sealant was used. The advantages of the PT-Bond coating include the improved long-term stability of the resulting surface functionalization and the provision of functionality for covalent bonds. When the adhesive adheres without covalent bonds, moisture/heat exposure rapidly causes the vulnerable dipole-dipole interactions between the hydrogen molecules of the substrate and the urethane/urea segments of the adhesive to break down. This would be fatal, particularly for a dryer where components are exposed to high temperatures and moisture over many years. Plasma treatment creates covalent, aging-resistant bonds and also makes it possible to dispense with conventional solvent-based primers.
Plasma replaces almost all primers in industrial production
For almost any manufacturing process that requires surface pretreatment, the shift to plasma surface treatment can provide a better, less expensive, and safer solution. The advantages:
• Near complete elimination of toxic solvents and primers is possible
• Surface cleaning and application of functional nanocoatings in a single step
• Surface treatment for long-term stable adhesive bonds and functional coatings
• Nanocoatings (PlasmaPlus) for generating specific surface properties such as hydrophobic, anticorrosive, or adhesion-promoting
Plastics often react to the solvents of the primers with stress cracks. On the other hand, however, no aging-resistant adhesion build-up can take place without dissolving the plastic. This dilemma for primers is solved by plasma, since no solvents are used in this case. Especially highly non-polar plastics such as PP, EPDM, or those that are very hard to dissolve benefit from plasma treatment. This is especially true for plastics with surfaces that can be modified by radical grafting. Plasmatreat's precision processes and ISO 9001:2015 certification ensure high quality and reliable reproducibility.
Plasmatreat Academy: The knowledge pool for plasma processes
For further education and knowledge transfer, Plasmatreat has created the Plasmatreat Academy. It focuses on the exchange between plasma-focused research and science, users of plasma systems in a variety of industrial sectors, and Plasmatreat employees with their experience in solving surface problems. The goal is to continually push the boundaries in the use of plasma processes for surface treatment so as to be able to offer economical, industry-relevant processes.