The impact of water on the statistical adhesion strength upon self-bonding onset at a polyester–polyester interface

Earlier, a very limited ability of dry poly(ethylene terephthalate) (PET) to self-bonding (autohesion) has been revealed both in the amorphous and semi-crystalline phase states. One of the possible ways to improve self-bonding at PET–PET interfaces is the addition of a plasticizer (water) between the contacting PET surfaces. Most interestingly seems to investigate this phenomenon at the critical autohesion conditions, at the autohesion onset temperature (T_a^onset), i.e., at the lowest temperature at which self-bonding between the contacting PET samples was still observed but below which was not. For this purpose, the thick bulk films of amorphous and semi-crystalline PET, abbreviated as amPET and crPET, respectively, were held in contact in a single lap-shear joint geometry, amPET with amPET and crPET with crPET, at T_a^onset = 74–84 °C (amPET) and 90 °C (crPET) for a predetermined period of self-bonding time of 30 min at a small contact pressure of 0.4 MPa providing physical contact at molecular level. As a result, two types of the amPET–amPET and crPET–crPET autohesive joints (AJs) have been formed: (i) without the addition of a plasticizer (water) at the interface between the two contacting PET samples and (ii) with the addition thereof onto the bottom PET sample prior to contact. As-formed AJs were shear fractured in tension at ambient temperature, and their lap-shear strength has been measured. In order to statistically reliably characterize the autohesion onset process of amPET and crPET, the statistical distributions of the adhesion lap-shear strength developed at both dry and wet amPET–amPET and crPET–crPET interfaces (totally, at four interfaces) have been analyzed in the framework of the Weibull model. The impact of water on the statistical parameters calculated, Weibull modulus and scale factor, at early stages of self-healing of the amPET–amPET and crPET–crPET interfaces has been discussed.