Straw fibres from barley hybrid lines and their reinforcement effect in polypropylene based composites

In order to investigate the role of plant hybridization in barley on the overall thermomechanical performance of polymeric composites containing their by-products, straws from one self-pollinated (Quench) and two F1 hybrid lines of barley (Tatoo and Zoo) were collected and characterized from the chemical (FTIR), structural (XRD), thermal (TGA) and mechanical (tensile) point of view, and used as reinforcements in a commodity plastic, such as polypropylene. XRD characterization confirmed a crystallinity index (CrI) equal to 57.59 % for the self-pollinated Quench straw, while the two hybrid lines showed slightly higher values (CrI of 61.55 % and 62.29 % for Tatoo and Zoo, respectively). TGA characterization showed an improved thermal stability for the cellulose fractions of the hybrid lines (Zoo and Tatoo) in comparison with the Quench straw, while FESEM investigation of the longitudinal external surfaces and cross sections showed a not uniform dense structure with variable thickness for the three lines, which contributed to a variable mechanical performance of the cereal straws. We found that hybrid lines possess slightly higher Young's modulus and tensile strength in comparison with self-pollinated Quench. Different mechanical response of the stem led to a higher stiffness for PP composites reinforced with hybrid lines, while tensile strength was not significantly affected by the barley straw line. The incorporation of 5 wt% of MAPP coupling agent improved both tensile strength and modulus of the composites, due to an increased interfacial adhesion between the barley straw fibres and the PP matrix.