New Methods and Efficiencies in Aerospace Thermoplastic Composites

The evolution of aircraft structural components is an innovative space, however the shift towards thermoplastic composites has only gained significant momentum in recent years. 

While these composites have long been employed on the leading edges of wings, it has only been within the last decade that manufacturers have also used thermoplastics for critical control surfaces, those that must stay intact for an airplane to land safely.

Why the hesitation? Perhaps manufacturers simply developed a comfort level with thermoset composites over the past 40 years, and saw no reason to change. 

Thermoplastics Were More Expensive – Until They Weren’t

The constant trend to optimize costs of raw materials and related processing expenses has accelerated the application of thermoplastic composites as alternatives to thermosets.

Though thermoplastics were long considered to be more expensive and more difficult to process, recent advancements in composite production and processing have made them a viable option in a wider array of aerospace applications, including cabin interiors, wing boxes and fuselage panels.

Thermoplastics do not need chemical crosslinking for solidification, and can be easily formed under sufficient heat and simply solidified by cooling to maintain their shapes at speeds much faster than curing of thermosets. Plus, they can be welded, which eliminates the need for bonding and riveting, and contributes to the ongoing quest to build lighter aircraft.

Finally, thermoplastics also have an unlimited shelf life (before curing) and can be recycled after cure. Gradually, that capability will likely allow for repair to thermoplastic parts while airplanes are still in the field, resulting in additional maintenance cost savings. 

The Promise of Plasma and UV Surface Preparation Methods

Surface treatment of a thermoplastic substrate is necessary to modify surface chemistry and improve wetting. This is typically accomplished through solvent cleaning, etchants such as chromic acid and other methods. But physical pretreatment is the most optimal option, and plasma surface treatment is preferred by many in the aerospace industry, as it provides stronger and more stable surface energy enhancement.

Another physical method is ultraviolet (UV) irradiation technique. A recent UV surface treatment technique developed at Henkel Aerospace offers a significant improvement in to adhesion strength within a shorter processing time.

In a recent study, three film adhesives with high-temperature cure systems were used as bonding adhesives for preparing mechanical test specimens, including fracture toughness and tensile lap shear specimens: LOCTITE EA 9696, EA 9695, and EA 9658. Two paste adhesives were also tried, including a room temperature cure adhesive, LOCTITE EA 9394, and an elevated temperature cure paste adhesive, LOCTITE EA 9394/C-2. For secondary bonding trials, LOCTITE EA 9895 WPP was used as a wet peel ply.

A surfacing film (LOCTITE EA 9845 LC) with copper mesh was used to evaluate the effect of the pretreatments on bonding strength of PEEK substrates and epoxy-based surfacing films.

Plasma treatment was done with an Open-air Plasma system with a rotation jet at atmospheric pressure. UV surface treatment was done by using a specific UV excimer lamp, and variation in UV intensity was done by adjusting the distance to the lamp and exposure time.

The results were as follows:

• Polarity and wettability of the composite surface were significantly improved by the formation of new functional groups, which formed covalent bonds with the epoxy adhesives.
• Adhesive chemistry positively impacted the proficiency of each treatment method. UV irradiation showed better performance for PEEK bonding with LOCTITE EA 9658, while plasma activation proved more efficient with LOCTITE EA 9695. However, both treatments improved adhesion strength.
• An optimization in the adhesive curing rate is favorable to achieve strong adhesion strength, likely by controlling chemical bonding between adhesive and new functional groups formed on the activated substrate.
• Plasma treatment leads to almost identical strength for co-bonding and secondary bonding.
• The new tailored UV pretreatment method resulted in very strong bonds to the thermoplastic PEEK substrate, comparable with plasma activation. However, peak efficiency strongly depends upon the chemistry and characteristics of the adhesive. 

Surfacing Film Applications

^{Interview with Florian Furlani, Technical Key Account Manager for Airbus, explaining the surfacing film technology and the broad surfacing film Henkel portfolio.}

Surface pretreatment of thermoplastic composites is essential for adhesive bonding, cosmetic applications and painting. To meet both aesthetic and environmental requirements, such as lightning strike protection, a surface film is applied to provide the required surface properties.

The constant trend to optimize costs of raw materials and related processing has accelerated the adoption of thermoplastic composites as alternative to thermoset composites and aluminum.

The advances in adhesive bonding and surfacing illustrated by this study may continue to increase confidence in the design and manufacturing of advanced aerospace composite structures. 

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