4.1.2. Resistance Heating

Resistance heating follows Joule's Law, as shown by Equation (4), where *P* represents Watts of heating, *I* is the applied current (in A), *V* is the applied voltage (in V), and *R* is the electrical resistance (in Ohms) [54].

$$P = IV = \frac{V^2}{R} \, , \tag{4}$$

As Equation (4) shows, for any given voltage the amount of heating is controlled by the resistance of an object. Therefore, the lower the resistance, the more heat it will generate. Moreover, under a given voltage, thicker films would heat up more because, as shown by Equation (5) below, the resistance of an object will decrease with a larger cross-sectional area:

$$R = \rho \frac{L}{A} \, \text{s}$$

where *ρ* is the resistivity, *L* is the length (in mm), and *A* is the cross-sectional area (in mm<sup>2</sup> ). Examples of thermal profiles measured for nanocomposite films containing 15 wt.% and 20 wt.% MWCNT are shown in Figure 7. The maximum temperature generally increased with film thickness, CNT weight fraction and applied voltage (Equation (4)). In some cases, slight deviations from this trend were expected based on the large standard deviations seen in Figure 6a and resulting resistance values. At 15 wt.% MWCNT, maximum temperatures of 58.0 ◦C, 78.7 ◦C and 108.0 ◦C were obtained at 10 V, for 0.06 mm, 0.25 mm and 0.50 mm thickness, respectively. On the other hand, at 20 wt.% MWCNT, maximum temperatures of 80.2 ◦C, 96.1 ◦C and 116.0 ◦C were obtained at 10 V, for 0.06 mm, 0.25 mm and 0.50 mm thicknesses, respectively. Figure 8 shows a composite of the measured temperature profiles during the entire Joule heating experiment, when voltage was increased from 2 V up to 10 V for all film thicknesses. Maximum temperatures were obtained at 10 V with 101.6 ◦C, 102.8 ◦C and 120.2 ◦C for 0.06 mm, 0.25 mm and 0.50 mm film thicknesses, respectively.

For the GF/PP adherends and MWCNT/PP films used in this study, their melting temperature (*Tm*) was measured by differential scanning calorimetry (DSC) in a previous study [36]. The adherends' *T<sup>m</sup>* was 150 ◦C, while the *T<sup>m</sup>* of the MWCNT/PP films varied between 141 ◦C and 149 ◦C. For the purpose of disassembly, it is expected an interface temperature close to, or slightly above, this range of temperature should be reached through the energy director (MWCNT/PP film). As previously mentioned in Section 4.1.1, the bond line thickness is expected to be equal to 0.2 mm at most. Therefore, an applied voltage above 10V would be required for disassembly experiments, based on the trends observed in Figures 7 and 8.

**Figure 7.** Temperature profiles of MWCNT/PP nanocomposite films at different input voltages and thicknesses: (**a**–**c**) 15 wt.% MWCNT, 0.06 mm, 0.25 mm and 0.50 mm thicknesses, respectively; and (**d**–**f**) 20 wt.% MWCNT, 0.06 mm, 0.25 mm and 0.50 mm thicknesses, respectively.

**Figure 8.** Representative temperature profiles measured for 25 wt.% MWCNT/PP films when voltages from 2 V to 10 V are applied for three minutes each.
