The effect of elastomer polarity on the morphology and mechanical properties of high-density polyethylene (HDPE) and scrap rubber powder (SRP) composites containing a polar or a non-polar elastomer was investigated. The morphological structure of the HDPE/SRP/elastomer composites was studied by scanning electron microscopy (SEM), dynamic mechanical analysis (DMA) and interfacial tension (γAB). The results show that in the HDPE/SRP/elastomer composites, two types of morphological structures were observed, either an encapsulation of the filler by elastomer or a separate dispersion of the phases. In the HDPE/SRP/non-polar ethylene–octylene copolymer (POE) composites, some POE encapsulates SRP particles and some other POE disperses uniformly. However, in the HDPE/SRP/polar ethylene–vinyl acetate copolymer (EVA) composites, SRP and EVA disperse separately. Both the impact strength and elongation at break are higher in the HDPE/SRP/POE composites than in the corresponding HDPE/SRP/EVA composites.