The Aerospace and Defense Industry is experiencing a market boom right now. Investments in the aerospace industry are an increasingly significant part of the world economy. Last year, almost $1 trillion was spent worldwide with about $400 billion spent directly on aircraft and space systems manufacturing in the United States alone. None of this is slowing down anytime soon.
The reasons for a renewed emphasis on aerospace are multifaceted in scope. This growth is creating some interesting trends in the manufacturing of the products used in this sector. Among these trends are continual improvements in materials used to construct aircraft. Lightweighting, or decreasing the heaviness of components, has become imperative as more aspects of the vehicles are turning to hefty electronics for fuel efficiency. Also, lighter aircraft in general can be safer and faster.
Additive manufacturing has stepped in to help cut costs in the production of these parts and expand the possibilities of lightweighting applications. Additive manufacturing, or 3D printing of components, has introduced new advanced materials to the aerospace industry. Printing on-demand large and small components is saving the aerospace industry loads of money in storage, waste and manufacturing time.
Parts such as wall panels, air ducts, and even engine components are increasingly being produced using additive manufacturing technologies. As the applications demand higher performance of these printed parts - such as higher strength, lower weight and more complex geometries - new materials have had to be developed that can rise to the occasion.
Polymers such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) are thermoplastics being used to create these heavy-duty, lightweight parts.
Thermoplastics are plastic polymer materials that become malleable when heated and rigid when cooled. They can be melted and remolded over and over without changing their chemical properties. PEEK and PEKK are particularly robust thermoplastics because of their strong mechanical properties as well as their extreme resistance to high temperatures. So they are strong, reliable, lightweight, low-outgassing, versatile and cost-effective alternatives to aluminum or other polymers in aerospace applications. Sounds like a winning combination!
What manufacturers need to be concerned with when using PEEK and PEKK is ensuring the surfaces are sufficiently prepared in a way that will guarantee adhesion that is as strong as the material itself. Often, surface preparation methods used on one material do not transfer well to others. If, for example, the production line is set up to clean and abrade aluminum surfaces then it is not set up for thermoplastics.
Adhesion is reliant upon a particular chemical composition on the material surface so that the adhesive will interact and bond with that surface at the top few molecular layers. It’s a very thin stratum that matters when talking about adhesion concerns. Materials have different innate surface chemical properties and they also interact with their environments differently. The treatments and preparations need to appropriately compensate for these variations.
If you are able to abrade or wipe clean an aluminum surface and get good adhesion it means that you sufficiently changed the chemical composition of the aluminum surface. Using the same procedure on a thermoplastic will not yield the same results because metals have an innately higher surface energy than polymers do. Surface energy is a way of talking about how reactive the surface of a material is. The more reactive the surface, the better the adhesion. A highly reactive surface is said to have high surface energy.
So the surface energy of thermoplastics needs to be increased and abrasion is not the method to get it there. Plasma treatments are an excellent way to prepare a polymer surface for adhesion because, with the right amount of treatment, the polar component of surface energy (the component with the most impact on adhesion) can be raised.
Plasma treatment is being widely used in production lines that utilize thermoplastics like PEEK and PEKK, but since they are a relatively new technique, the effect they have on surfaces isn’t fully understood.
Plasma treatment works, essentially, by bombarding a surface with excited atoms to create an oxidized and highly reactive surface.
It is crucial to understand exactly how long to expose the surface to this treatment though. Even slightly extended exposure can go beyond oxidation and lead to what is called chain scission, or a weakening of the polymer chain of molecules. Prolonged exposure causes scission which creates carboxyl groups. Chain scission fractures the polymer surface, creating activated species (like carboxyl groups) but not the kind that deliver good adhesion performance. Applying adhesive to a fractured polymer surface can make the adhesive stick well to this fractured surface, but not well to the bulk substrate. This can cause poor bonding performance and, what’s possibly worse, to the naked eye it can appear like an interfacial failure, leading manufacturers down an incorrect problem solving path.
To prevent scission and weak bonds, manufacturers need to have a clear picture of the chemical composition of their surface before treatment and then verify that the treatment level was sufficient but not more than necessary. In order to make sure the full strength and versatility of thermoplastics is realized, treatment has to be monitored and optimized. This is a Critical Control Point that is often left uncontrolled with no quantifiable means to validate how chemically clean the surface is before and after treatment.
This kind of process control can be implemented by pairing the plasma treatment with a surface quality inspection. The more data you can gather about the quality of your surfaces, the better your chance at building a predictable and superior adhesion process.
For more information about implementing steps to ensure your surface preparation is ideal for your process, download our eBook: Predictable Adhesion in Manufacturing Through Process Verification