Morphology of calibrated foams

The thin foam strands treated above are too small for mechanical measurements. Hence, a calibration unit was applied to obtain calibrated foams, which enable preparation of larger test specimens. In this section the morphology of these calibrated foam samples is discussed and compared to the morphology of the foam strands.

Figure 83 depicts densities of all calibrated foams investigated, which were measured at the department of Polymer Engineering at the University of Bayreuth using the buoyancy method. Densities of all calibrated foams are in the range between 0.24 and 0.33 g/cm³. This means a drastic density enhancement caused by the compression of the hot foam during calibration compared to densities of foam strands without calibration, which are between 0.08 and 0.18 g/cm³. Neat stabilized i-PP without any cell nucleating agents features a density of 0.33 g/cm³ after foam calibration, which is the highest of all calibrated foams. All samples comprising cell nucleating agents feature lower densities. For these samples, correlations with foam density after calibration are observed neither for the chemical struc-ture nor the concentration of additive. Concluding, by calibration of foams, large samples for mechanical testing were successfully produced, which feature densities in a narrow range.

The comparability of foam densities after calibration is highly beneficial for later mechanical testing, as it prevents variation of mechanical properties due to density differences.

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Figure 83: Foam densities of calibrated foams of neat i-PP (black), i-PP containing talc (red) and i-PP containing BTAs 5 (green), 6 (blue) and 7 (magenta). Symbols represent averages and error bars represent standard deviations of three measurements each. Adapted with permission from ref. (Mörl et al. 2019)¹⁹¹. © (2017) The authors.

Next, the microscopic morphology in the bulk of calibrated samples was investigated, being another property which could alter mechanical properties. Figure 84 shows SEM images of selected examples of calibrated foams. The images show that the cellular structure is still intact after calibration. In contrast to the foam strands discussed above, cells of calibrated foams feature clear shape anisotropy, i.e. they have been deformed during calibration.

Foam cell size was taken as a quantitative measure for the bulk morphology. Figure 85 shows average cell diameters of calibrated foams measured from SEM images at the department of Polymer Engineering at the University of Bayreuth. After calibration, foam cell diameters are in the range between 27 and 68 µm. This is quite the same as for

non-text

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Figure 84: SEM micrographs of calibrated foams. Materials are neat i-PP (A) and i-PP with 5000 ppm of talc (B), 300 ppm of BTA 5 (C), 3000 ppm of BTA 6 (D) and 4000 ppm of BTA 7 (E).

calibrated foams, which featured diameters in the range of 27 to 73 µm. Moreover, after calibration no correlation between cell diameter and cell nucleating agent is observed anymore. By contrast, a tendency towards smaller cells with increasing additive content was visible for foam strands (see Figure 80). Besides, standard deviations have increased, meaning that foam cell sizes are less homogeneous after calibration. This is attributed to the mechanical deformation of the foam during the calibration process. Summarizing, average cell sizes after calibration are comparable to the ones prior to calibration, which indicates that the calibration process does not alter the inner foam structure to a great extent.

Up to now, all compounds of stabilized i-PP and cell nucleating agents have been treated as macroscopically homogeneous materials. To obtain information about the single nano-objects present in the materials, calibrated foam samples were searched for additive nano-objects at the SEM. In doing so, foams without further treatment as well as etched foams were investigated. For the latter, etching was done in an aqueous solution of KMnO4, H2SO4 and H3PO4 for 1 h. SEM images of etched and non-etched foams after sputtering with Pt are shown in Figure 86. To increase the chance of finding objects, only foams comprising the maximum concentration applied in foaming of each additive were investigated this way.

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Figure 85: Average foam cell diameters of calibrated foams of neat PP (black), PP containing talc (red) and i-PP containing BTAs 5 (green), 6 (blue) and 7 (magenta). Symbols represent averages and error bars represent standard deviations of at least 100 measured foam cells each. Overlapping symbols are slightly shifted for the sake of clarity. Data have already been published in ref. (Mörl et al. 2019)¹⁹¹. © (2017) The authors.

Neat stabilized i-PP (1A) exhibits a rough surface texture with spikes prior to etching. This surface texture is removed by etching (1B), resulting in a smoother surface. The rough surface may stem from polymer crystallization processes at the gas/polymer interface. At the sample comprising 5000 ppm of talc, nano-platelets covered by a thin i-PP layer were observed without etching (2A). After etching, similar platelets lying at the surface without any cover layer were apparent (2B). These structures are identified as talc nano-platelets.

For all three samples comprising BTAs, nanofibers were present in the non-etched samples (3A-5A). These fibers all seem to be partly covered by the i-PP matrix, while other parts pierce closer to the surface and can therefore be observed. In most cases it cannot be decided from SEM images whether visible fiber segments are still covered with a thin i-PP film or are in direct contact with cell gas. Nevertheless, at least for the right fiber in image 5A, a direct contact with air is visible for the fiber’s end. This direct contact is required for the applied cell nucleation concept: It is based on cell nucleation at the interface between i-PP and the nano-object. When a cell nucleated this way grows, the nucleation site is supposed to stay uncovered at the foam cell surface. Yet, reliable conclusions on the occurrence of the proposed cell nucleation mechanism from these pictures are not possible.

After etching, all samples comprising BTAs show structures which are understood as impressions from the above mentioned nanofibers (Figure 86 3B-5B). Since BTA fibers are completely removed from the surface in the etching process, these impressions are supposed to be relicts of these fibers. The impressions only feature a small length reflecting the formerly uncovered area of the former fibers, which could exclusively be removed by the etching solution. In image 3B at the top end of the impression, a brighter area indicates that the fiber was longer than the imprint. Of course, fibers which are totally buried deep in the i-PP matrix cannot be displayed by this method. Also, due to the curved morphology of the foam surface, only a very small area of each cell could be investigated with the SEM, which is nevertheless assumed representative for the whole cell.

Concluding, it is possible to detect BTA fibers at the surface of foam cells. The use of non-etched samples is preferable to non-etched ones, as those allow direct observation of fibers, whereas after etching only a negative impression remains. Moreover, etching removes some i-PP from the cell surface and so prevents observation of fiber structures at the cell surface, which are the most interesting ones for drawing conclusions on the nucleation process.

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Figure 86: SEM micrographs of calibrated foams prior to (A) and after (B) etching. Materials are neat i-PP (1) and i-PP with 5000 ppm of talc (2), 300 ppm of BTA 5 (3), 3000 ppm of BTA 6 (4) and 4000 ppm of BTA 7 (5).

Poorly visible nanostructures are marked with arrows.