## 4 Discussion  
The presence of residual solvent in polymeric materials can considerably affect its curing process and chemical structure, thus altering its mechanical properties as well. In samples designed to observe this phenomenon, TGA confirmed that the presence of varying weight percentages of IPA in the adhesive affected its decomposition. Figure 2 shows how a higher wt.% of IPA caused higher weight loss from 100-320 \({}^{\circ}\)C, which is due to the IPA likely influencing the curing process of the adhesive [6,53]. This affected its chemical structure by possibly increasing the presence of lower molecular weight chains, which underwent thermal degradation earlier. FTIR also verified the presence of IPA within the structure of the adhesive which may have affected the crosslinking process during curing. Similar behavior was observed by [PERSON] et al. when they investigated the effect of acetone on the properties of an epoxy resin [53]. Lap shear testing showed that the presence of trapped solvent in an adhesive can heavily decrease its adhesion properties. This is consistent with the results reported by [PERSON] et al., in which they showed how the addition of 14 vol. % of xylene solvent into an epoxy adhesive decreased its adhesion strength by 35% [55]. They also reported that the samples with this same amount of solvent experienced a decrease of 60% in their tensile strength compared to the pristine resin samples. Figure 4b demonstrates that adhesive failure was obtained but no major visual changes in the failure surfaces occurred for the different sample types. This emphasizes the need for a non-destructive evaluation method, since performing destructive testing to obtain the properties of adhesives is time-consuming and costly. Ultrasonics can help in performing in situ testing during the manufacturing process,  
Figure 8: (**a**) Longitudinal time delay between the excitation and output signal vs. 1/frequency; (**b**) shear time delay between the excitation and output signal vs. 1/frequency. and, as shown in Section 3.6, in obtaining the elastic properties of polymers if their density is known. It was demonstrated that ultrasonics can be used to monitor the curing process of an epoxy adhesive by evaluating changes in sound speed due to variations in its chemical structure. The first test shown in Figure 5 of three different baseline adhesive samples shows that the ultrasonics setup could repeatedly be used to monitor the kinetics of polymer curing. This helped determine that the fluctuations in the curing behavior of the samples containing residual solvent (Figure 6) were due to the IPA preventing an ideal curing rate of the adhesive. Previous work has shown that the tensile strength of this adhesive with respect to cure time starts converging between curing hours 8 and 10 [56; 57], meaning that the sound speed is more sensitive than tensile strength to the gelation process and early crosslinking between the polymer chains of the curing adhesive. Repeated laboratory testing reflected that both the mixing technique and the adhesive's temperature could influence its curing process. In addition, evaluating the sound speed in fully cured adhesives that are manufactured properly can serve as an in-field quality control method during the lifetime of those materials. The evaluation of the elastic properties of the epoxy adhesive using ultrasonics was corroborated with tensile testing values, validating this non-destructive evaluation method. Modeling the curing process using the Hill equation helped in achieving a better numerical comprehension of how the curing rate of the adhesive was affected by the trapped solvent. This model can predict how improper manufacturing influences the properties of materials by correlating the rate constant \(k\) and shape parameter \(\theta\) to the strength of the polymer. A decreasing trend was observed for the rate constant although no variation was found for the shape parameter.