## 5 Adhesion Improvement Assessing
### Interfacial Shear Strength Evaluation  
The interfacial adhesion was determined by single fiber fragmentation tests, carried out on model composites consisting of a single GF embedded in a PP matrix. Microcomposite samples were prepared by compression molding. Fibers were carefully aligned between two PP films and in turn sandwiched between two stainless steel sheets. Microcomposites were prepared using untreated and treated fibers with 0.5%, 1.0%, 1.5% and 2.0% of hydroxy-\(\alpha\)-olefin, and they were named _PP/F (% hydroxy-\(\alpha\)-olefin)/PP_. Single fiber fragmentation tests were performed on model composites consisting of a single fiber embedded in a polymeric matrix. It was measured at room temperature by using a custom-made apparatus as shown in Figure 13 [50,51]. The mean fiber length, Ls, was measured by means of an image analyzer system, as proposed by [PERSON] _et al._[52]. An example of the fiber failure appearance is reported in Figure 14 for PP/F0/PP and PP/F1.0/PP. For the untreated fibers (Figure 14a), the interfacial debonding appears to be the dominant failure mechanism, thus indicating low-adhesion conditions. On the other hand, a different failure behavior is observed for microcomposites with fibers previously treated with 1.0% of hydroxy-\(\alpha\)-olefin. Here, a  
Figure 12: Weibull plot of untreated and MAO-treated fibers [50]. radial matrix crack or mixed radial-conical cracks occur (Figure 14b). This behavior is an indication of a somewhat better fiber matrix-adhesion, as reported by [PERSON] _et al._[51, 53]. From Figure 14 also clearly shows that the number of fiber fragments increases with the interfacial adhesion, resulting in a shorter critical length, as summarized in Table 4.  
Figure 14 shows the appearance of microcomposites after the fragmentation test for samples prepared using untreated fibers and treated fibers in optimum condition (1% hydroxy-a-olefin). These photographs, taken with optical microscopy equipment, indicate that in the untreated fibers, the segments are long, the remaining space between segments is shaped \"cylindrically\" and the crack appears to have spread into the matrix (Figure 14a). Moreover, in the case of fiber microcomposites copolymers, the segments are significantly shorter and the space segment-segment has a different shape, similar to that of a double truncated cone whose vertex is set off from the ends of the fiber segments. Additionally, the crack is spread widely in the matrix (Figure 14b). In the literature three possible failure modes from fragmentation tests are reported. Debonding failure, where the crack propagates parallel to the interphase, is the typical case of a very poor adhesion with low interfacial resistance. If the resistance is higher, the crack can propagate to the array before the fiber falls away. This behavior manifests itself in a disk perpendicular to the rupture of the fiber, and interfacial resistance is typical of socks. The third case, when adherence is very good, a cone-type fracture is observed with a vertex at both ends of the broken fibers. Importantly, the behavior intermediate between those described presents as a combination of these patterns of fracture [53]. Figure 15 shows these models schematically. Clearly, the results obtained (shown in Figure 14) describe two extreme cases. No adherence seems to be the dominant failure mechanism in the case of PP/F0/PP microcomposites showing poor interfacial adhesion, on the contrary, the samples that have broken polymerized matrix-radial cone type fractures from the interface of the fiber indicate a marked improvement in adhesion. In the same table, the interfacial shear strength, calculated in accordance to Equation (1), is also reported. The adhesion markedly increases when fibers with _in-situ_ polymerization are considered. For microcomposites obtained with polymerized fibers using 1% of hydroxy-a-olefin, an ISS value more than two times higher than that of the untreated fibers is observed.