Plasmatreat systems to clean composites, increase surface energy for bonding
(Top left, counterclockwise) Plasmatreat offers Openair-Plasma systems that operate at atmospheric pressure and low-pressure systems in vacuum enclosures for a variety of composites including glass fiber/epoxy laminates in circuit boards. Its PlasmaPlus technology can covalently bond nanocoatings onto surfaces as tie layers for dissimilar material bonding as well as dielectric and galvanic corrosion barriers. Source (All Images) | Plasmatreat
Founded in 1995,Plasmatreat (Steinhagan, Germany) is a family-owned company producing roughly 800 systems per year from plants in Germany, the U.S. and China. It has sold more than 10,000 plasma treatment systems worldwide. Although initial adoption and use has been mostly in metals, plastics and semiconductors, Plasmatreat technology has been used in composites for years and is growing.
Offering automated, reliable and homogeneous cleaning and surface energy activation in seconds, Plasmatreat systems can target large or focused, localized areas. Its technology enables optimized bonding for paints and coatings, as well as for composite skin-stringer assemblies in wingskins and multi-material bonding (such as hybrid metal and plastic composites) common in automotive. But this technology can also deposit nanomaterials and atomized liquids in 1- to 2-micron-thick layers for various types of protection, including dielectric barriers, as well as other functionalities — increasingly important for next-gen materials in aircraft, space and energy generation.
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How plasma works
Plasma is ionized gas. “Our systems use cold plasma,” explains Ryan Robinson, senior R&D engineer for Plasmatreat. “We start with a high-voltage arc but immediately stretch and dilute it with a high flow of gas, such as air or nitrogen, for example, so that it’s just slightly above room temperature when it hits the parts.”
“We’re basically putting a high voltage into clean, dry air to generate the plasma,” adds Sarah Montrowl, business development leader, aerospace for Plasmatreat. “As the plasma passes over the surface, it’s breaking the carbon backbone of contaminants like grease, wax and dirt and then evaporating them, leaving a pristine surface.”
plasma treatment increases the surface energy of composites
Plasma treatment increases the surface energy of composite surfaces for improved adhesion and bond strength.
The plasma also changes the surface free energy — for example, from hydrophobic to hydrophilic — which improves its wettability for adhesives and coatings. Traditionally, a water droplet test has been used in composites to indicate a properly prepared surface for bonding. After traditional grinding or hand sanding and then solvent wipe of a carbon fiber/epoxy laminate, for example, a water drop on that surface should be flat with a contact angle <40° versus a hydrophobic surface where water drops will bead up with a contact angle >90°.
“However, our systems use only air and electricity, so there are no volatile organic compounds [VOCs], harsh chemicals or solvents, and no harmful byproducts,” says Robinson. “It’s also an automated process, alleviating the manual labor and inconsistencies that come with manual surface prep, and much more sustainable than most traditional methods.” This includes chemical etching and anodizing in metals.
Plasma also forms functional hydroxyl-, carbonyl- and carboxyl- groups to increase surface energy which increases adhesion and bond strength, says Montrowl. “That’s a different mechanism than just using abrasion. It enables chemical covalent bonds with the adhesive or coating due to the plasma functionalization versus a mechanical bond you find with physical roughening.” For example, treatment of an aerospace-grade carbon fiber/epoxy laminate can boost surface energy from 28 to 68 millinewtons/meter. We’ll come back to this later.
System types, scalability, applications
Plasmatreat sells two basic types of systems based on the pressure used. Low-pressure systems apply plasma jets within a vacuum chamber while its trademarked Openair-Plasma systems use jets at atmospheric pressure. These can be inside an enclosed cabinet or part of a larger automated system and frequently integrated with cartesian or six-axis robots.
“For our low-pressure systems, the standard chamber is comparable to a mini fridge,” says Robinson. “There’s also a large capacity system (roughly 1,854 × 1,422 × 610 millimeters) for parts the size of a car bumper. You can pack quite a few large parts in there.”
A new extra-large capacity system is in development for composite wing sections, honeycomb panels and wind turbine rotor blades, notes Montrowl. “It’s narrower and very long, offering huge time savings and reliability for these large components that are currently being hand sanded.”
Openair-Plasma systems can also be scaled. They comprise of a generator, plasma control unit and plasma jet. “We offer systems that can run one jet up to 16 jets from that single generator,” says Montrowl. “We also provide a range of jet diameters for treating localized or large areas. Our largest diameter so far is 135 millimeters.”
And how fast can these treat a composite surface? “Typically, in seconds,” says Robinson. “We can also vary the speed, but a common setting is 150 millimeters/second.”
A wide spectrum of composite materials are being treated by Plasmatreat systems, says Montrowl. “We are currently working with carbon fiber and glass fiber composites, different types of epoxy composites, all types of honeycomb-cored parts but also thermoplastic composites which have become a large focus for us.”
Increased bonding without damaged fibers
“What has come to light, particularly as we move into more advanced thermosets and thermoplastics, is that these surfaces can be really challenging for bonding, especially when they have a nonpolar nature,” says Montrowl. Polymers and plastics are often nonpolar, meaning the surface has neither a positive nor negative charge to enable covalent bonding. “During plasma treatment, we are inserting oxygen groups and functionalizing the surface, but we aren’t exposing fibers,” she explains. “That’s one of the chief complaints we hear about hand sanding is that you don’t want to degrade the material you’re trying to bond to. There is no ablation with our cold plasma, so there’s also a cost saving from removing human error.”
This lack of mechanical or thermal damage to the surface also enables work on delicate materials, notes Robinson. “We can perform quick treatments, for example, to 200-micron-thick polymer membranes before they apply specialized coatings for filtration, with very good bonding.”
test of surface preparation results on CF/PEKK composites
University of Washington test results on carbon fiber/PEKK composites using various surface preparation methods. Source | Ref. 1
Tests performed in a coatings study at the University of Washington (Seattle) compared carbon fiber/PEKK composites with various types of surface preparation. Samples made using a Plasmatreat system showed the lowest contact angle (highest wettability), highest functionality and lowest paint area removed.1
Boeing test results comparing surface energy on CFRP and GFRP composites
Surface energy test results on carbon- and glass fiber-reinforced epoxy composites comparing surface preparation methods. Source | Ref. 2
In another set of tests performed by Boeing R&T (Seattle, Wash., U.S.), Universidad Carlos III (Madrid, Spain) and Brighton Technologies Group (Cincinnati, Ohio, U.S.), carbon- and glass fiber-reinforced epoxy resins also showed a significant increase in total and polar surface energy compared to solvent wipe and grit blasting.2
lap shear tests show 40% higher bond strength with plasma treatment
Lap shear tests show 40% higher bond strength when carbon fiber-reinforced polymer, adhered to aluminum with a two-part epoxy adhesive, uses plasma treatment versus no surface preparation.
This increase in surface energy and functional groups on the composite surface leads to higher bond strength. For example, tests Plasmatreat conducted on carbon fiber-reinforced polymers showed a 40% higher lap shear strength versus an untreated surface and the preferred cohesive failure — where the adhesive breaks within itself — instead of adhesive failure, where the adhesive separates from the bonded surface.
“We’ve also seen test results from the University of Texas where fibers treated with our system exhibited better adhesion to matrix resins. This has become an interesting area of work for several of our customers” says Montrowl.
Plasma-deposited coatings, multifunctional composites
Not only can Plasmatreat systems tailor surfaces for certain coatings, they can actually apply coatings. “In our low-pressure systems, we introduce liquid chemicals that turn into a vapor as they enter the chamber,” explains Robinson. “We call this plasma-enhanced chemical vapor deposition, and it enables us to deposit and covalently bond nanocoatings — layers that are 10-700 nanometers thick — onto a surface.”
“We can also do this using our atmospheric jet systems,” says Montrowl, “which is our PlasmaPlus technology. It’s typically a second step performed after first using the plasma jet to clean and activate the part surface, and we can deposit functional coatings using different precursors. Typically, we have used it as a tie layer for adhesive bonding dissimilar materials such as composites to metals or plastics.”
“But we can also apply siloxanes as a barrier coating for increased resistance to corrosion in metals,” says Robinson. ”We’re using the same approach to prevent galvanic corrosion with composites in bondline applications, and to apply dielectric barriers onto glass fiber/epoxy laminates and other materials in electronics and circuit board applications.”
“Because this is a nanocoating, it’s nonintrusive for part designs,” adds Montrowl. “And it has now been added into the specifications for a lot of these designs and processes.”
Growing applications
Plasmatreat is seeing increased growth in larger systems. “The industry is switching to large robots for painting,” says Montrowl, “and our large-area jets are being integrated into these. So, they’ll clean, activate and then paint rather than hand sanding all those big structures. After years of development work, several customers have now added our large-area jet systems into their process specifications.”
But she notes the use of large low-pressure chambers is also growing. “We’re seeing more interest in wingskins, engine nacelles and wind blades — bigger parts that are just high enough in volume so that labor-intensive methods such as sanding or mechanical surface preparation are no longer a good fit. These chambers are also becoming the preferred method for parts with a significant amount of geometry like honeycomb or hard-to-reach, narrow features.”
Higher volumes are a trend Plasmatreat is seeing across industries. “We have always been heavily involved in high-volume automotive production,” notes Montrowl, “where our systems fit well into highly automated lines. We have also seen the electronics industry switch over from batch treatment to inline solutions as their production volumes have increased. Our Openair-Plasma systems are now used inline to treat circuit boards to ensure proper adhesion of conformal coatings, which allows flexibility in selective cleaning and per piece workflow. And we’re seeing similar trends in aerospace.” As advanced air mobility matures and OEMs increase production rates for single-aisle jetliners, manufacturers here are also seeking new technologies to help achieve higher production volumes on automated lines.
“That’s where we see a lot of growth in the future,” says Robinson. “We’re working on many other inline applications, for example, we have a specialized jet that helps coatings for manufacturing of solar cells. We’re always looking for partnerships where we can develop new applications. So many industries are looking for higher performance but also improved process efficiency and sustainability. We have a technology that can help.”
References
1 “Comparative study of surface preparation for paint adhesion on CF-PEKK composites: Plasma, chemical, and flame treatment,” by Nandi, Ankush, et al. Applied Surface Science 669 (2024): 160533.
2 “Surface modification of composite materials for adhesive bonding,” by Noemí Encinas, Juana Abenojar, Miguel A. Martínez: Universidad Carlos III; Brietta Oakley and Giles Dillingham: Brighton Technologies Group; Kay Blohowiak and Marcus A. Belcher: The Boeing Company.