Both polypropylene (PP) and polyethylene terephthalate (PET) constitute a significant portion of post-consumer waste. To improve the recycling of immiscible PP/PET blends, a compatibiliser should be utilised. The steady shear flow properties of unmodified and modified PP/PET blends having up to 50 wt.% PET were investigated and compared in this study. Three types of PPs with different flow properties were used to ascertain the influence of the matrix on the blend's rheology. The effect of modification on the rheological properties was evaluated in two ways - firstly, the addition of 1 wt.% of maleic anhydride (MA), and secondly, the use of already modified polypropylene. According to the morphological observations, an improvement in compatibility was found in both cases. The shear viscosity and the first normal stress difference were measured using a rotational cone and plate rheometer at 265°C (when both PET and PP are molten), and 245°C (when only PP has melted). Completely different behaviour was observed under these two temperature conditions. At 265°C, the shear viscosity decreases with PET content in the blend, while at 245°C it increases, thus recalling the behaviour of particle-filled systems. The addition of maleic anhydride affects the shear viscosity in various ways; a decrease, an increase, and some almost unchanged values were obtained. Concerning the first normal stress difference, an even more complex situation occurs, and the effect of modification by MA is also ambiguous. Furthermore, the deviations from the log-additivity rule were evaluated in terms of the shear viscosity and the first normal stress difference. From the results, it can be supposed that PP-X/PET samples were compatibilised successfully, and strong interphase interactions could be expected. Finally, the yield values of shear stress determined at 245°C showed a generally increasing tendency with increasing PET content.
The effect of surface treatment on the rheological properties of 20 and 40 wt.% short glass fibre reinforced polypropylenes (GF/PP) and of the same composites containing maleic anhydride modified polypropylene (GF/mPP/PP) were investigated. Glass fibres were surface-treated by aminosilane coupling agent (ASC, γ-aminopropyltriethoxysilane). Steady state shear and dynamic properties in the molten state were measured using a cone and plate type rotational viscometer.
Surface treatment increased the rheological properties of the molten composites. This trend was more pronounced for composites containing modified PP. In the case of the GF/PP materials the increase was monotonous for all measured functions, while in the case of the GF/mPP/PP ones, the parameters representing elasticity showed peaks at 0.5 wt.% ASC. The Cox-Merz empirical law was found to be applicable to untreated GF/PP and GF/mPP/PP composites at low shear rates (angular frequencies), where the materials showed Newtonian plateaux.
As for ASC surface treated materials, GF/mPP/PP followed the same trend. On the other hand, GF/PP exhibited a higher complex viscosity than the steady state shear one over the whole range of angular frequencies (shear rate) covered by the experiments. Surface treatment enhanced apparent yield stress, as did increasing the glass fibre content. The effect of the latter was more pronounced in the case of the GF/mPP/PP series. The viscosities of the re-mixed composites became lower than those of the original ones, and the difference between untreated and ASC treated systems was detectable only in the case of the complex viscosity of the GF/mPP/PP sample.
The increasing amount of post-consumer waste leads to a search for effective recycling methods. Compatibilisation of immiscible blends of polypropylene (PP) and polyethylene terephthalate (PET) represents one such novel approach that allows recycling of significant scrap materials without any need for separation steps. The intention of this paper is to investigate the dynamic viscoelastic properties of unmodified and modified PP/PET blends. To study the effect of the matrix, three polypropylenes differing in flow properties were chosen. The compatibilisation was carried out by both one- or two-step reactive in situ techniques. In the former case, the addition of 1 wt.% of maleic anhydride, and in the latter, polypropylene supplied already modified by the producer were employed. The measurements were performed by using a rotational cone and plate rheometer at temperatures of 265 °C and 245 °C. Since PET is not melted at the lower temperature, completely different behaviour is observed in the two cases. The rheological functions generally decrease with PET content at 265 °C (conventional blend behaviour), while they increase at 245 °C, in a manner reminiscent particle-filled systems (including the yield behaviour). The positive effect of compatibilisation can be deduced not only from the results of the morphological study (dispersed phase size reduction, strengthened interfacial adhesion), but also through increased values of the viscous and elastic characteristics, which suggests enhanced interactions between the blend components. Comparing steady shear and dynamic viscoelastic properties, the extent of deformation applied during both types of measurements leading to different material responses should be considered.
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