Demonstration of Hybrid Effect in Single Fiber Pull-Out Tests for Glass/Cellulose-Reinforced Polypropylene with Different Fiber–Matrix Adhesions

In hybrid fiber reinforcement, the combination of glass and regenerated cellulose fibers is a promising combination because the different properties of the fibers can be combined. The properties of the regenerated cellulose fiber in combination with the absorption of energy by fiber pull-outs can thus significantly increase the toughness of the composite in the event of failure, while the glass fiber significantly increases the stiffness and strength due to its properties. In this study, the interaction of the two fiber types in a composite is demonstrated by fiber pull-outs. For this purpose, the fibers are embedded in a PP matrix and simultaneously pulled out. Different bondings of the fiber by, e.g., coupling agent and/or a pretreatment of the regenerated cellulose fiber, were also investigated. The results show that each type of fiber has a characteristic force–deformation curve, and the hybrid reinforcement is a combination of both curves. The use of a coupling agent leads to an increase in the interfacial shear stress from 4.5 to 7.5 MPa. A treatment of the regenerated cellulose fiber by UV light further increases the interfacial shear stress to 11 MPa.