Polyethylene terephthalate (PET) is a semi-aromatic thermoplastic polyester, which is widely used in many commodity and also engineering applications [1–3]. Its extensive usage is mainly due to its superior mechanical properties, high chemical resistance, good barrier properties, high thermal stability, low production cost, and recyclability [4–9]. While the global production of PET was over 56 million tons in 2016, it increased to ~ 70 million tons in 2020, confirming a dramatic increase in PET usage in various applications [10]. Therefore, the increase in its accumulation in the nature turns to be a serious global concern [11, 12]. According to European Union (EU) new regulations, plastic bottles should contain at least 25% recycled materials by the end of 2025 and this percentage will increase up to at least 30% by 2030 [13, 14]. Therefore, it is crucial to reduce the growing waste of PET in the nature and ensure its reuse in the industry due to both commercial and ecological concerns.
Among several recycling routes, melt reprocessing is a practical and widely applied method to recycle PET due to its low cost and little adverse environmental impact [12, 15]. However, melt reprocessing of PET could cause chemical, mechanical, thermal, and oxidative degradations. These result in reduced molecular weight and viscosity of PET, which eventually suppress its processability. This is while the physical and mechanical properties of PET would also be depressed [16, 17]. These undesired degradations could restrict the direct reuse of recycled PET (R-PET). The use of chain extender (CE) has been offered as a practical and facile breakthrough to compensate the molecular degradation of PET during reprocessing and hence the suppression of the final properties of recycled polymers [18–23].
Depending on the chemical nature and number of available functional groups of CEs, linear chain extension and/or branching of PET molecules could be observed. So far, several CEs such as bis-oxazolines [24], pyromellitic dianhydride (PMDA) [22], organic phosphites [25–27], isocyanides [28, 29] and epoxides [4, 5, 29–35] have been proposed to extend the molecules of polyesters. Among these CEs, multifunctional epoxides better known under the trade name of Joncryl® ADR are the most commonly used CEs to enhance the melt properties of PET [36].
In one of the early studies [37], various recycled polymers were chain extended with Joncryl 4368 at different contents. It was observed that this oligomeric CE has a wide gel-free processing window that could enhance the melt properties of highly degraded polymers. According to Raffa et. al. [38], 1.5 wt.% of Joncryl 4368 could sufficiently enhance the melt viscosity of R-PET. In another study, it was found that the molecular weight of R-PET increased more efficiently when a mixture of Joncryl 4368 and Joncryl 4370 was used in a reactive blending with post-consumer R-PET [39]. Such a mixture of Joncryl grades was also reported by Duarte et al. [40]. Xiao et al. [41] also illustrated that the reactive extrusion of PET with Joncryl 4370 induced a higher elasticity than that with PMDA. In a recent study conducted by Härth et. al. [42], molecular structure and rheological properties of PET were analyzed after chemical modification with PMDA and Joncryl 4368 at different concentrations. While PMDA-modified PET resulted in a linear and a tree-like macromolecular chains, the Joncryl-modified PET induced a strongly branched molecular structure.
Melt blending with other compatible high temperature resistant polymers is another route to compensate the inferior properties of R-PET. Due to its good processability, fast crystallization, and similar molecular structure, polybutylene terephthalate (PBT) is recognized as one of the most promising thermoplastics to be melt blended with R-PET. This is to resolve the suppressed mechanical, thermal, and rheological properties of R-PET. In this context, the enhanced melt properties and increased crystallization rate of R-PET/PBT blends could accordingly improve the poor processability of R-PET [43–49]. While PET and PBT are known to be miscible in their amorphous phases with a single glass transition temperature (Tg) [42, 43, 47, 48], depending on the processing and hence the cooling profile they may be either miscible or immiscible in their crystalline phases due to the different crystallization rates of PET and PBT [43].
Although the properties of virgin PET and PBT blends have extensively been discussed [44–52], there is a limited research on the R-PET and PBT blends [43, 49]. Baxi et al. [49] incorporated R-PET into virgin PET/PBT blends at different concentrations and achieved enhancements in mechanical properties and degree of crystallinity of PET/PBT (40w/60w) blends containing 60 wt.% of R-PET. In a recent study, Nofar et. al. [43] explored the phase miscibility in R-PET/PBT blends at different ratios while investigating mechanical and thermomechanical properties of these blends. Nonetheless, thermal stability and especially rheological properties of the R-PET/PBT blends have not been properly discussed in the literature. Moreover, the combined effect of melt blending R-PET with CE and PBT on the thermal stability and rheological properties of R-PET has not been clearly disclosed. In this study, the thermal stabilization and rheological properties of R-PET with the addition of Joncryl and melt blending with PBT are investigated. The synergistic effect of melt mixing R-PET with Joncryl and PBT on the thermal stabilization and melt properties of R-PET is addressed through small amplitude oscillatory shear (SAOS) rheological analysis. Fourier-transform infrared (FTIR) spectroscopy is also used to better understand the structure of the melt processed samples.