**3. Prediction of Heat-Affected Zone for Disassembly Experiments**

Despite the low thickness of the bond line (≤0.50 mm) and adherends (<1.90 mm), a lag was expected between the surface temperature and the actual temperature at the welded interface. A simplified 3D thermal analysis was carried out in the SolidWorks Thermal Simulation module to predict the temperature at the surface of the adherend (as shown in Figure 3) for various interface temperatures. The goal of these analyses was to provide a range of surface temperatures that would guide the design of the disassembly experiments, based on material properties from the suppliers and found in the literature.

Figure 4a shows the boundary conditions for the thermal finite element analysis (FEA). The thickness of the bond line was 0.1 mm (half-thickness of MWCNT/PP interface) and the GF/PP adherend was 1.8 mm thick. A forced air convection coefficient of 12 W/m<sup>2</sup> ·K was selected for the room in which tests were to be carried out. It was applied to all adherend surfaces in contact with air. The contact between ED and adherend was defined as "Bonded". The temperature at the interface was set at three values: 120 ◦C, 140 ◦C or 160 ◦C, based on the melting temperature of GF/PP adherends and MWCNT/PP films, between 140 ◦C and 150 ◦C [36]. Due to the orthotropic behavior of the adherends (UD layup), two thermal conductivity values (*ky*, *k<sup>x</sup>* = *kz*) were estimated using the rule of mixture shown in Equation (2) and Equation (3):

$$k\_{\mathcal{Y}} = (1 - V\_{\mathcal{G}F})k\_{\mathcal{P}\mathcal{P}} + V\_{\mathcal{G}F}k\_{\mathcal{G}F} \tag{2}$$

$$\frac{1}{k\_{\rm x,z}} = \frac{(1 - V\_{GF})}{k\_{\rm PP}} + \frac{V\_{GF}}{k\_{GF}} \tag{3}$$

where *VGF* is the glass fiber volume fraction, *kPP* is the thermal conductivity of polypropylene (in W/m·K) and *kGF* is the thermal conductivity of glass fibers (in W/m·K). The material properties for the GF/PP adherends are listed in Table 1. The heat capacity, *C<sup>p</sup>* (in J/kg·K), was calculated based on the rule of mixture, as described in Equation (2). The MWCNT/PP films were assumed to exhibit isotropic properties with random carbon nanotubes orientation and distribution. The main thermal properties are listed in Table 1 with the corresponding references in the literature. The nanocomposite films' thermal conductivity was estimated to range between 0.55 W/m·K and 0.65 W/m·K based on [47,48], to account for MWCNT weight fraction and potential variations in dispersion.

**Figure 4.** (**a**) Boundary conditions for finite element analysis used for prediction of temperature profile through the thickness of GF/PP joint. Symmetry was assumed along the ZY plane; and (**b**) Example of 3D thermal plot with location of plotted results at the mid-plane, along x-direction at center of bond line.

**Table 1.** Main estimated GF/PP adherend and MWCNT/PP films thermal properties used in FEA. Refer to Figure 4a for coordinate system.


a: Suppliers' specifications sheet (PolyOne and Professional Plastics); b: [5]; c: [47,48].

Figure 5 shows the through-the-thickness temperature profiles along line A at the cross-section labeled in Figure 4b. The results for 15 wt.% MWCNT/PP film at the bond line are presented, but no significant differences were found for the range of *kCNT/PP* values in Table 1. The surface temperature at the center point of the overlap is 112.8 ◦C, 131.3 ◦C and 149.9 ◦C for an interface temperature of 120 ◦C, 140 ◦C and 160 ◦C, respectively. Given a temperature gradient around 10 ◦C, as well as the assumptions and simplifications made for thermal analysis, it was estimated that the disassembly experiments should be carried out when the surface temperature of the GF/PP adherend reached 110 ◦C, 130 ◦C and 150 ◦C to adequately capture the behavior of the heated joint.

**Figure 5.** Predicted through-the-thickness temperature profiles along line A at the cross-section labeled in Figure 4b when interface is set at a temperature of 120 ◦C, 140 ◦C and 160 ◦C. The bond line material was 15 wt.% MWCNT/PP.
