PSI - Issue 53

João Soeiro et al. / Procedia Structural Integrity 53 (2024) 367–375 Soeiro et al. / Structural Integrity Procedia 00 (2023) 000–000

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to continue the separation process in the regions of weaker adhesion, until a second area of stronger adhesion is solicited. Such non-uniform adhesion behavior could be attributed to the (i) partial presence or (ii) complete absence of a chemical bond, which is known to enhance the adhesion strength between di ff erent polymers. The former would indicate that the observed behavior is a mirror of the micro mechanical interlocking that takes place in the surface, while the latter suggests that the chemical bond is not consistent. The force-displacement curves for the parts with 5 wt% of HDPE-g-MAH reveal a di ff erent adhesion behavior compared to the parts without the additive. The curves show an initial gradual increase in force with increasing displacement, but the rate of increase is slower, due to seemingly enhanced adhesion, the resulting maximum force is delayed, being reached at a later displacement. Unlike the curves for the parts without HDPE-g-MAH, there is no abrupt drop in force following the maximum point. Instead, the force decreases gradually, suggesting a more uniform adhesion strength across the adhered surface. Furthermore, there are no multiple peaks of force, only the maximum, which is slightly higher. The sample parts with no compatibilizer displayed an average maximum force value of 32 ± 2 N, while the parts with 5 wt% of the compatibilizer displayed an average maximum force value of 37 ± 2N.

Fig. 5. Influence of HDPE-g-MAH compatibilizer on the force-displacement curves obtained from the pull out tests.

3.2. SEM analysis on polymer-metal samples

Figure 6 shows a cross-section of the metallic insert with (insert moulded) HDPE in baseline conditions, (manufactured under the previously specified operational injection conditions with the insert at room temperature and mold temperature of 50 ◦ C, through insert molding, without any surface treatment of the metallic insert. The higher magnification SEM images present two distinct types of adhesion failure that are observed at the polymer-metal interface. The first type, referred to as type 1 (shown in figure 6b) is characterized by a complete lack of adhesion, where there is no observable contact between the polymer and the metal. The second type of adhesion failure, type 2, shown in figure 6c exhibits a di ff erent failure mechanism. Here, there is an apparent good adhesion between the polymer and the metal. However, polymer fracture seems to occur. This type of failure indicates that while there is some level of adhesion achieved, it is not su ffi cient to maintain the structural integrity of the polymer layer at the interface. The micrographs of the parts with abrasion and flamed surface treatments inserts are presented in figure 7. The SEM images reveal that the surface treatments were e ff ective in promoting good contact between the polymer and metal. The analysis suggests that there were no occurrences of type 1 adhesion failures, characterized by a total lack of adhesion. Instead, only type 2 adhesion failures were observed, where the initial layer of polymer in contact with the metal separated from the rest of the polymer. Concerning the inserts where the surface was flame treated (refer to figures 7a and 7b), it is also important to mention that the majority of these occurrences were localized on the vertical planes of the connector tracks, where the flame treatment was not as meticulously applied due to the small dimensions