Single Versus Double Screw Extrusion: Impact on Mechanical Performance and Crack Resistance for a Virgin, Recycled, and Blended Polypropylene

To evaluate the influence of extrusion on material performance, three processing methods, single extrusion (1x Ex), double extrusion (2x Ex), and double screw compounding (C), were investigated for virgin polypropylene (vPP), recycled PP (rPP), and a blend containing 10% rPP (rPP10). Calculated thermal stresses and shear during processing revealed that 1x Ex imposed the lowest load, 2x Ex doubled thermal and shear stress, and C introduced more than 16.8 times higher shear. Rheological measurements revealed a slight viscosity increase in vPP after 1x Ex, due to branching, followed by a decrease under higher stresses. In contrast, rPP exhibits viscosity reduction independent of processing method. Melt flow rate increased with processing stresses for vPP and rPP10 samples, while the rPP samples remained unaffected. Standard thermal, FTIR, and mechanical tests detected no significant differences, but oxidation onset temperature indicated stabilizer consumption. Crack resistance was highly sensitive, while vPP remained unaffected, rPP already exhibited a tenfold decrease in crack resistance after 1x Ex, quantified by the number of cycles until failure. In rPP10, vPP compensated rPP degradation, but higher stresses reduced crack resistance. The results highlight that conventional tests may overlook early degradation, whereas rheological analysis can indicate material performance, but requires careful interpretation.