Morphology and apparent density 138

SEM micrographs showing solid skin/foamed core taken from different sections and directions of the selected conditions are outlined in Figure 5.14 and Figure 5.15.

Figure 5.15. SEM micrographs of PP 20GF foamed plates taken in TD direction.

Fibers in surface layers seem to be contained in a parallel plane to the main surface of the plates, whereas in the foamed core follow the pattern of the fountain flow [293]. Cells near the skin are elliptical and oriented in the filling direction and circular in TD direction, as previously stated, indicating that they were nucleated during the filling stage and deformed by shear flow.

Once again, bubbles distributed around glass fibers indicate that heterogeneous nucleation was the predominant mechanism of PP 20GF foaming. The morphological parameters are summarized in Table 5.8. As seen with ABS before, the solid skin became thicker as the distance from the injection gate increased (MD-A, MD-B and MD-C), because molten polymer reached the cavity ends at lower temperatures and solidified faster. Due to

this build-up in the solid skin thickness, sections at the end of the cavity were denser than those near the injection gate (Table 5.9). In TD direction, however, the solid skin thickness remained practically constant in all different studied sections (TD-A, TD-B and TD-C) for each condition. Furthermore, this surface layer was thicker in foamed samples with 10% of weight reduction, as compared to that of 20%-reduced weight foams, because of higher expansion of the foamed core with higher contents of blowing agent.

Cell density was kept in an order of magnitude of 106 cells cm-3 in all cases. Previously, it has been already discussed that, the higher gas content introduced to get a 20% of weight reduction would increase cell nucleation. However, if growing force of cells is larger than the strength of cell walls, cell coalescence occurs and results in larger bubbles but similar cell density to that of foamed samples with 10% of weight reduction. As a consequence, CDI parameter increased in the lighter foamed series. However, Figure 5.16 points out analogous cell size distributions in every analyzed sample with 90% of them smaller than 100 µm, despite C5 foamed series contained some bigger cells.

Table 5.8. Morphological parameters in different sections of PP 20GF foamed plates. Condition

No.

Section Skin thickness (mm)

Cell density (cells cm-3₎

Cell size range (µm) CDI C1 (10%) MD-A 0.52 4.8·106 _{2-100 1.28} MD-B 0.71 3.1·106 8-188 1.63 MD-C 0.81 2.5·106 6-180 1.94 TD-A 0.77 1.0·106 _{9-147 1.39} TD-B 0.76 3.5·106 6-135 1.36 TD-C 0.77 1.8·106 3-145 1.57 C5 (20%) MD-A 0.42 2.9·106 7-252 2.08 MD-B 0.59 3.4·106 4-240 1.89 MD-C 0.71 5.5·106 7-203 1.82 TD-A 0.59 4.7·106 _{7-243 1.96} TD-B 0.58 1.6·106 2-248 1.80 TD-C 0.59 7.0·106 6-215 1.79

Figure 5.16. Cell size distribution of different sections of PP 20GF foamed samples with a) 10% weight

reduction; b) 20% weight reduction.

Figure 5.17. Cell size and cell density results of PP 20GF foamed plates simulated with Moldex 3D® software. Figure 5.17 collects the results of cell size and cell density obtained with Moldex 3D® simulation software. Although the increase in the amount of blowing agent for foaming with 20% of weight reduction, the software predicted no changes in cell density and slightly lower cell sizes, concurring with the range of bubble diameter that contains around 85% of cells.

Table 5.9. Apparent density and fiber content measured in different sections of the PP 20GF square plates. Condition No. Apparent density (g cm-3₎ A B C D E Solid 1.03 ± 0.01 1.03 ± 0.01 1.03 ± 0.01 1.03 ± 0.01 1.02 ± 0.01 C1 (10%) 0.79 ± 0.01 0.86 ± 0.01 0.82 ± 0.01 0.86 ± 0.01 0.88 ± 0.01 C5 (20%) 0.67 ± 0.01 0.78 ± 0.01 0.73 ± 0.01 0.78 ± 0.01 0.80 ± 0.01 Condition No. Fiber content (%) A B C D E Solid 20.5 ± 0.1 20.4 ± 0.1 20.3 ± 0.2 20.3 ± 0.1 20.1 ± 0.3 C1 (10%) 20.3 ± 0.2 20.6 ± 0.1 20.6 ± 0.1 20.4 ± 0.1 20.6 ± 0.1 C5 (20%) 20.5 ± 0.3 20.2 ± 0.4 20.3 ± 0.3 20.2 ± 0.2 20.5 ± 0.1

On the other hand, Table 5.9 points out that fiber content is in the range of 20.4 ± 0.2%, being the fiber concentration in solid plates of 20.3 ± 0.2% and 20.5 ± 0.1% in the polymer in pellets form, as previously aforementioned. That is, the filler content remained invariant despite the decrease in apparent density from 1.03 ± 0.03 g cm-3 _{(solid plates) to 0.88 ± 0.01 g}

cm-3 _{and 0.79 ± 0.01 g cm}-3 _{(10% and 20% of weight reduction, respectively) due to foaming}

process.

Another important morphological feature of fiber-filled composites is the orientation and distribution of the fibers. In these materials, the orientation of the fibers has more effect on the mechanical response than the molecular orientation [294]. Additionally, it can influence shrinkage and warpage of the part and compromise its dimensional stability. Fiber orientation is mainly dependent on the processing conditions [275]. Pictures of fiber orientation in the surface and in the middle plane of molded samples taken from Computed Tomography technique are illustrated in Figure 5.18, Figure 5.19 and Figure 5.20. To better understand this phenomenon, the flow pattern obtained from simulation is included. In the center plane of the plates, flow was diverging and induced transverse alignment of fibers to filling direction [295] as can be observed in section C-Core of Figure 5.18. Only near the walls, at surfaces and sides of the samples, shear stress caused a higher orientation in the flow direction or, as in this case, no obvious preferential alignment in the surface layers.

Figure 5.18. Filling flow pattern and Computed Tomography pictures of PP 20GF solid plates.

Figure 5.20. Filling flow pattern and Computed Tomography pictures of PP 20GF foamed plates (20% wt. red.). These fiber orientation and distribution patterns work are in agreement to the first researches carried out by several authors [296-299]. According to the type of load and ratios of skin/core of the different positions in the molded plates, the mechanical response will be higher or lower in magnitude. Gong et al. [300] discussed about a minimum length of the glass fiber in order to effectively bear stress in foamed PP. From the CT analysis carried out in this work, an average length of 740 ± 150 µm of the glass fibers for all solid and foamed samples was calculated. Since the maximum cell size was about 250 µm, these fibers were long enough to pass through the cells and reinforce the polymer matrix.