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 will slow down anytime soon.
The reasons for a renewed emphasis on aerospace are multifaceted in scope. This growth is creating some interesting trends in manufacturing 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 vehicles are turning to hefty electronics for fuel efficiency. Also, lighter aircraft, in general, can be safer and faster.
New Applications for Advanced Materials
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 saves 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.
Thermoplastics Have What It Takes
Polymers such as polyetheretherketone (PEEK) and polyether ketone ketone (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!
Manufacturers need to ensure the surfaces are sufficiently prepared to guarantee adhesion as strong as the material itself when using PEEK and PEKK. The surface preparation methods used on one material often 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.
Know Your Materials on a Molecular Level
Adhesion relies upon a particular chemical composition on the material surface, so 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 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 gain high-performance 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 higher surface energy than polymers. 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.
Treat Your Thermoplastics Right
Thermoplastic materials require increased surface energy for optimal adhesion. Abrasion techniques are not recommended for this purpose, as they can damage the material. Plasma treatment offers a superior solution. By carefully selecting treatment parameters, plasma can selectively increase the polar component of the surface energy, which plays a critical role in adhesion performance.
Plasma treatment is a growing technology widely adopted in production lines for advanced thermoplastics like PEEK and PEKK. While highly effective, ongoing research continues to refine our understanding of its optimal use. The process functions by bombarding the surface with energetic atoms, promoting oxidation and creating a highly reactive surface. However, precise control of treatment duration is critical. Excessive exposure can lead to chain scission, a phenomenon where polymer chains break down. This weakens the material and creates different reactive groups (carboxyl groups) compared to those ideal for adhesion. Bonding an adhesive to a fractured surface can produce a 'good' initial adhesive bond to the disrupted layer but not necessarily to the underlying bulk material. This translates to poor overall bonding performance, potentially mimicking interfacial failure and leading to troubleshooting inefficiencies.
Process Monitoring is Critical
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. Treatment has to be monitored and optimized to ensure the full strength and versatility of thermoplastics are realized. 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 of 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: The Future of Manufacturing: A Guide to Intelligent Adhesive Bonding Technologies & Methodologies.
