## 3 Results
### Mechanical and Morphology Analysis of the Curaua/Composites
However, the statistical analysis showed a significant difference between the PHB matrix and CWM 20 wt% (tensile strength) and flexural properties; only the composites of CF 10 wt% showed no significant difference. The filling with 20% curaua led in two cases to an improved strength ((a) untreated curaua and (b) fibers treated with chlorite and mercerized). Unlike the PLA composites, the filling with washed and mercerized fibers did not improve the PHB matrix's strength. This led to the assumption that the treatments did not significantly improve the adhesion between the PHB matrix and the fibers. However, the addition of curaua fibers made PHB more flexible. Statistically, the PHB matrix showed a significantly different flexural modulus compared to all composites. Thus, PHB, a crystalline and fragile material, has become more flexible [45]. The mechanical analyses were correlated with an SEM. Representative image for each composite group are illustrated to reduce the number of pictures in the text. Figure 5 shows the fracture surface of the PHB polymer and the PHB-containing composites. As the most significant differences in strength were found for filling degrees of 20%, the displayed images are from these groups of composites. Figure 5: SEM micrographs for PHB (**A**); composite containing 20% of curaua fibers (untreated) (**B**); composite containing 20% of curaua fibers (treated and mercerized) (**C**); composite containing 20% of curaua fibers (treated with chlorite and mercerized) (**D**). Figure 5A shows pores found in the pure PHB polymer and less evident in the composites. Such pores usually significantly influence strength. The appearance of the pores foretells that the manufacturing method either did not run with adequate parameters or was generally insufficient to generate defect-free polymer/composite specimens. The fracture surfaces of the composites are evaluated considering three features: (A) the fiber distribution, (B) fiber pull-out behavior, and (C) the fractured surface. As visible in Figure 5B, the untreated fibers occur in two different ways on the surface. First, big fiber bundles that protrude out of the surface are visible. Their existence confirms the result of the initially realized morphologic analysis after the chemical treatments, which show only slight fiber defibration. Besides the fiber bundles, many loose fibers are visible on the fracture surface. This result was surprising, as separating single fibers from the main bundle was only expected for the composites with chemically treated fibers. The length of the protruding fiber bundles reaches up to 200 \(\upmu\)m. Such protrusion length indicates low adhesion between fibers and matrix, and the fiber could be pulled out easily. The fracture surface of the composites containing washed and mercerized fibers showed similar properties. Rough and irregular can describe the surface. The amount of single, loose, and pulled-out fibers increased compared to the untreated fiber. The amount of visible fiber bundles generally decreased. Nevertheless, they are still present. A black hole, which initially accommodated a fiber bundle pulled out during the tests, is visible in the image's upper half. The length of the pulled-out single fibers also indicates that the adhesion between matrix and fiber was low. Unlike the described fracture surfaces, the composites with fibers treated with chlorite and mercerized showed a laminar and plane surface. Pulled-out fiber bundles are visible on the left edge. Their occurrence reveals none of the applied fiber treatments: (A) significantly improved the adhesion between fibers and (B) led to the defibration of the fiber bundles. However, no protruding single fibers are visible on the surface. This can be related to two effects: First, the adhesion could be improved in a way that the pull-out of single fibers does not happen, even though the pull-out of fiber bundles cannot be prevented. Second, the chemical treatment increased the brittleness of the single fibers.