2.2. Preparation of MXene Coating

To enhance the adhesion between the epoxy substrate and MXenes, a surface plasma treatment was performed using a Zepto Diener low-pressure plasma cleaner (Diener Electronic GmbH & Co. KG, Ebhausen, Germany) and a K1050X RF plasma cleaner (Quorum Technologies, Laughton, UK). The samples were treated under vacuum with no additional gases for 3 min with 100 W power at 40 kHz frequency. The water contact angle was reduced from 68.7° (for pristine epoxy) to 25.3° (for the modified epoxy surface). Another treatment was performed in a vacuum with an oxygen and argon enriched atmosphere for 3 min with 100 W at 13.56 MHz. The contact angle was reduced to 20.1°. After the epoxy surface plasma treatment, water-based MXenes were sprayed using a Sparmax HB-040 airbrush with a 0.4 mm diameter nozzle and a Sparmax DC-25X 2.07 bar compressor with 0.1 mL/s paint yield (Anest Iwata Sparmax Co., Taipei, Taiwan). The MXene coating was applied by spraying for 10 s at a distance of 15 cm, which naturally dried over 5–10 s at room temperature. Copper wires were directly soldered onto the MXene films using the commercially available electrically conductive polymer Protopasta (Protoplant, Inc., Vancouver, WA, USA). This polymer is made from a mixture of polylactic acid (PLA) (4043D PLA, Natureworks, Minnetonka, MN, USA) and carbon black (CB). The volume resistivity of the polymer is 30 Ωcm, and the melting point is 155 °C. The soldering needle temperatures were set to 200 °C, similar to the 3D printer nozzle temperatures. The soldered wires were additionally covered with a thin layer of silver paste. The distance between the silver paste contacts for both the epoxy and GFRP samples was 30 mm. The epoxy specimen sprayed with MXenes and a monitoring system (described in Section 2.3) are presented in Figure 1.

MXene-coated epoxy specimen and electrical resistance measurement system.