| Summary: | Flax and lignin were used to prepare hybrid polypropylene composites. The lignin contents were varied between 0 and 50 vol%, while the amounts of flax used were 0, 10, 20, and 30 vol%. The components were homogenized in a twin-screw compounder and then injection molded into tensile bars. Tensile and impact tests were used to characterize the mechanical properties, and the structures were studied by scanning electron microscopy. The simultaneous effects of reinforcement on composite properties were analyzed quantitatively with models to establish synergies or a coupling effect of lignin. The agreement between measured results and predicted values proved to be excellent. The models were also successfully applied to data published in the literature. Lignin increased the strength from 10 to 20 MPa, and a smaller amount of added flax increased the strength up to 30 MPa. Modeling also established the stronger reinforcing effect of flax; the corresponding B parameters for lignin and flax were 3.7 and 5.9 for strength and 1.0 and 5.2 for impact resistance, respectively. The two reinforcements acted independently; their effects were additive, and synergism could not be detected. Only quantitative analysis and modeling offer reliable information about the effect of components in hybrid materials.
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