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.