To upgrade the quality of TPE material, it was blended with recycled thermoplastic material in various amounts ranging from 10 to 50 weight percent (wt%). For a first series of tests, styrene–ethylene–butylene–styrene (SEBS) was mixed with recycled polypropylene (rPP). The addition of 10, 30, and 50 wt% rPP resulted in a minimal increase in the melting temperature of the new blend compared to the pure SEBS material, with the melting temperature rising from 153 °C (0 wt% SEBS) to 156 °C (50 wt% SEBS). However, due to the large addition of rPP to the SEBS material, the melting enthalpy increased by 300%, from 17 J/g to 50–54 J/g, depending on the rPP grade used. In other words, despite only a small increase in melting temperature, this new blend requires significantly more energy to melt compared to pure SEBS. Other thermal properties, such as the glass transition temperature and degradation temperature, remained nearly unchanged upon the addition of rPP. In addition to thermal properties, the rheological and mechanical properties of the new materials were also investigated. Capillary rheology showed that the MFR value of the material containing 50 wt% rPP is almost 16 times higher than that of pure SEBS TPE at 230 °C (0.65 g/10 min for pure SEBS and 10.31 g/10 min for the blend containing 50 wt% rPP). Furthermore, the addition of rPP leads to an increase in hardness (Shore A) of the new blends. The mechanical properties are also strongly influenced by the presence of rPP. Properties such as the Young’s modulus, maximum tensile strength, and compressive strength increase significantly with the addition of rPP. From these initial tests, it can be concluded that the mechanical properties of SEBS‑based TPE materials can be enhanced by adding recycled PP, without significantly affecting their thermal properties.
In addition to the upgrading tests performed on TPE materials, it was also examined whether rPP can be upgraded by adding (r)SEBS material. For this purpose, different grades of rPP were blended with various grades of (r)SEBS in different ratios. For the initial tests, the different grades of rPP and (r)SEBS were not mixed with each other in order to avoid unnecessarily increasing the complexity of the new materials. Concretely, new rPP-based materials were produced containing 5 to 10 wt% (r)SEBS. Regardless of which rPP grade and which (r)SEBS grade were used, several trends were observed during the characterisation of the new materials. The addition of (r)SEBS has virtually no influence on the melting temperature (160 °C) of the new compounds. The melting enthalpy (55–78 J/g) also remains nearly unchanged upon addition of (r)SEBS. The degradation temperature of the newly formed blends is slightly lower than that of pure rPP, which is explained by the fact that the degradation temperature of pure (r)SEBS materials is lower than that of rPP. In addition, the incorporation of (r)SEBS also results in a decrease in the glass transition temperature by a few degrees. Rheological experiments were also carried out on these new materials. From these, it was found that the MFR values (measured at 230 °C) of the blends are at most 5% higher than those of the pure rPP materialdes, pite the fact that the MFR values of the pure (r)SEBS materials are significantly lower than those of the rPP grades used. Furthermore, several mechanical properties of the new materials were determined. First, tensile tests were performed on the new blends, and the results were compared with those of the pure rPP materials. Analysis of the data showed that the tensile strength, flexural strength, and Young’s modulus of the new blends are lower than those of the pure rPP materials. However, the addition of (r)SEBS leads to a 14–28% increase in impact strength compared with the pure PP materials.
In a next phase, it will also be investigated whether recycled TPE material can be upgraded using a similar strategy. In addition, the recyclability of these newly developed blends will also be examined.
