Effect of hemp fiber length on the mechanical and thermal properties of polypropylene/SEBS/hemp fiber composites

The control of the fiber length is proposed as a low cost and eco-friendly alternative to the expensive chemical treatments of fibers in polymer composites. Different initial lengths of hemp fibers (HF), ranging from 1 to 4 mm, were obtained using automatic milling and manual cutting. The effect of HF length on the properties of composites was studied in a complex system consisting in polypropylene (PP)/poly[styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS)/HF composite. A reduction of HF average length of about 2, 2.5 or 4 times, depending on the initial fiber length, was observed after extrusion and injection molding. The reduction was greater compared to the results reported for PP/HF composites due to the presence of SEBS and injection molding step. Although the thermal stability of pristine HF was influenced by the intensity of the milling treatment, the effect of different fiber lengths on the degradation of composites was minor. A higher crystallinity was obtained in composites with longer fibers, which correlates to the increased stiffness. At room temperature, the storage modulus was increased by 82–90% in composites with longer fibers and by only 58% in the ones with a high proportion (90%) of “fines”. Therefore, the treatment of HF in a laboratory mill with adjustable die is a simple method to obtain the desired range of fiber length and a clear improvement of mechanical properties. The PP/SEBS/HF composites with a fiber length of more than 2.5 mm are promising materials for electric vehicles, where the reduced weight and environmentally friendly character are very important.