Spinnability and Surface Properties of Fibers Made of Recycled and Virgin Polypropylene

Strain-hardening cementitious composites (SHCC) are ductile concrete composites. Because of the incorporation of polymeric micro fibers, multiple cracking occurs under increasing tensile load in contrast to the brittle behavior of conventional concrete. This quality makes SHCC an interesting material for applications that require resistance to high-velocity impact. Mostly, polyvinyl alcohol fibers or high-density polyethylene fibers are used in SHCC, however, their high mechanical performance is accompanied by high costs resulting from the production by gel-spinning. In contrast, polypropylene (PP) fibers are produced by more efficient and less complex melt-spinning with comparably low energy consumption. They are widely used for other purposes in concrete already and can be a suitable alternative for the use in SHCC because of their good performance/ price ratio and alkali-resistance. On the other hand, their smooth surface and lack of active groups results in a poor wettability and insufficient interaction with the cement matrix, what usually results in lower SHCC mechanical performance. To overcome this issue, in this work a recycled PP (rPP) is used for the fiber melt-spinning. This combines the aspect of sustainability as well as the fact, that rPP shows potential to enhance fiber wettability because of a higher oxidation degree. The use of rPP could therefore be a way to improve the adhesion between fiber and matrix. However, the high shear stresses and temperatures during recycling by multiple processing may compromise the mechanical properties and limit the processability during spinning. Therefore, the aim of this work is to compare the surface properties of rPP and virgin PP (vPP) as well as their spinnability.