Textile industry produces a high amount of residues that, nowadays, are poorly managed. The majority of such wastes are dumped and landfilled. Among all the textile value chain, cotton yarning factories produce wastes in the shape of fiber flocks, with lengths smaller than 10 mm that prevent their reintroduction in the textiles manufacturing process. Nonetheless, such waste cotton flocks could be used as reinforcement for short fiber mould injected composites. This paper reports on the behavior of the interphase between the cotton flocks and a polypropylene matrix. It was found that the organic dyes present on the cotton flocks seem to affect the quality of the interphase in two ways: on the one hand by increasing the affinity between the cotton fibers and the matrix, and on the other hand by limiting the effect of the coupling agents. Micromechanic models are used to further research the quality of the interphase and the intrinsic properties of the composites.