Rheological study of recycled polypropylene–starch blends

Blends of recycled polypropylene (PP) and starch (S) with the compositions polypropylene 83 wt%–starch 17 wt% (PP83/S17) (blend 1a), polypropylene 68.8 wt%–starch 31.2 wt% (PP 68.8/S 31.2) (blend 2a) and polypropylene 89.5 wt%–starch 10.5 wt% (PP 89.5/S 10.5) (blend 3a) were synthesized. Maleated polypropylene (MAPP) was used as a compatibilizer. The compositions of the compatibilized blends were PP73/S15/MAPP12 (blend 1b), PP55/S25/MAPP20 (blend 2b) and PP85/S10/MAPP5 (blend 3b). The occurrence of a reaction between MAPP and starch was studied using Fourier transform infrared analysis. Thermal and rheological properties such as the complex viscosity, storage and loss modulus of the blends with a compatibilizer were found to be higher than those of the blends without a compatibilizer. The compatibilized and uncompatibilized blends, as well as recycled PP, were characterized using differential scanning calorimetry, thermogravimetric analysis and cone-and-plate rheometry. The storage and loss modulus values of blend 3b were observed to be the best. The best compatibilizing effect was exhibited by blend 3b at a loading of 5 wt% MAPP because this compatibilizer content yielded the highest complex viscosity and visco-elastic behavior. The presence of a functional compatibilizer enhanced the interactions between starch and recycled PP, which was confirmed by a rise in the melt viscosity, storage modulus and thermal stability. These blends were also characterized in terms of their water uptake by performing water absorption tests. Blend 2b containing 20 % MAPP was observed to absorb the maximum amount of water at 25 °C.