Research methodology

Seven series of specimens consisting of injection mouldings were prepared for the study. Table 1 shows the exact chemical composition of the specimens.

Table 1. Composition of the tested composites

No. PE-LD Malen E

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Specimens were ruptured to obtain fractured surfaces for microscopic observation. The fractured surfaces were cut off with a guillotine and taped to slides compatible with the microscope stage. The specimens were attached to the stage of the microscope, which was then placed in the Quorum Q150T ES sputter coater along with the specimens. There, in an argon atmosphere, a thin (10 nm thick) layer of gold was sputtered onto the surface of the specimens, which facilitated observation under the microscope (Fig. 3).

Fig. 3. Process of gold sputtering and the specimen after sputtering

Prepared in this way, the specimens were placed on a mobile and rotating microscope stage mounted on a retractable drawer (Fig. 3 and Fig. 4). Once the specimens were in the chamber and the chamber was closed, the air was extracted.

Fig. 4. Parts of Scanning Electron Microscope: microscope stage and image on the computer screen

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Observations were carried out in low vaccum (60 Pa). The distance of the specimen from the detector, the exact position of the detector above the specimen, and the parameters of the scan beam could be monitored on the screen (Fig. 4).

5. RESULTS

Figures from 5 to 11 show the results of microscopic observantions (SEM) of polymer composites containing halloysite nanotubes and an optional addition of a compatibilizer. Figure 5 shows the surface of pure polyethylene without additives. The surface is homogeneous without visible inclusions, even at large magnifications.

Fig. 5. Image showing polyethylene without the addition of nanotubes and compatibilizer

Figure 6 shows the fractured surface of a composite containing 2% of nanotubes, and Figure 7 is an image of a composite with 2% of nanotubes and 5% of compatibilizer. In both cases, nanotube agglomerates of various sizes (from several to a dozen μm) are visible on the polymer surface. Given the size of the nanotubes themselves, the size of the agglomerates is quite large.

Enlargements of Figure 6 show the way the agglomerates are embedded in the matrix. The polymer does not cling fast to their surface, which may indicate poor adhesion of the agglomerates to the polymer. In the next Figures (Fig. 8, Fig. 9, Fig. 10 and Fig. 11), the agglomerates are larger and more densely arranged with the increase of halloysite nanotubes in polymer matrix. It can be seen that they are abundant in places where the specimen has ruptured. A large concentration of nanotube agglomerates is observed in Figure 10. The enlargement shows that

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only a small amount of the polymer has penetrated into the interior of the nanotube cluster. The presence of such structures will most likely adversely affect the properties of the entire composite.

Figure 11 also shows large agglomerates of nanotubes, up to a dozen μm, at the edge of the fractured surface. The presence of structures of this size may also adversely affect the strength of the material.

Fig. 6. Image showing polyethylene with 2% addition of nanotubes and two enlargements of nanotube agglomerates

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Fig. 7. Image showing polyethylene with 2% addition of nanotubes and 5%

of compatibilizer

Fig. 8. Image showing polyethylene with 4% addition of nanotubes

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Fig. 9. Image showing polyethylene with 4% addition of nanotubes and 5%

of compatibilizer

One can see tiny needle-like structures that match the dimensions of the nanotubes as provided by the manufacturer. In most cases, however, the nanotubes are clustered into large agglomerates, sized several to a dozen μm. The last Figure 12 shows nanoparticle agglomerates mechanically separated from the polymer matrix from a cluster so large that it was visible to the naked eye. Small, needle-like structures can be seen, matching the dimensions of nanotubes provided by the manufacturer. Still, they mostly appear in big agglomerates of a few to a dozen µm. There is an analogy between the observed agglomerates and the nanotubes in Fig. 1.

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Fig. 10. Image showing polyethylene with 6% addition of nanotubes and enlargement of halloysite nanotube agglomerate

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Fig. 11. Image showing polyethylene with 6% addition of nanotubes and 5%

of compatibilizer

Fig. 12. Nanotubes agglomerates separated from polymer

To verify the results, the obtained images were compared to those obtained by other authors examining halloysite nanotubes dispersion with SEM. Figure 13 is a SEM image from paper [16], depicting the decomposition of halloysite nanotubes in a polilactic acid matrix. One can clearly see agglomerates of nanotubes of size and appearance corresponding to those obtained in this paper.

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Despite the presence of agglomerates of 20-30 µm in size, the authors of those papers define the dispersion level as good.

Fig. 13. Decomposition of halloysite nanotubes in a polilactic acid matrix, SEM image [16]

The image of halloysite nanotube cluster obtained in [17] can be seen in Fig.

14. The authors describe the impact of halloysite nanotube contents on EPDM properties. On the basis of this image, they describe the face and face-to-edge interaction of nanotubes and their influence on tensile strength and brittleness of the composite. They claim that the image allows to assume that the nanofiller was dispersed correctly in the polymer matrix. Surprisingly, the authors of [17] applied the magnification of merely 5000x.

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Fig. 14. Tensile fracture surfaces of EPDM/HNT nanocomposites with (a) 5 phr, (b) 10 phr, (c) 30 phr and (d) 70 phr (magnification 5000x) [17]

Fig. 15. Distribution of HNTs in HNTs–epoxy composites (10 wt.% HNTs) prepared by: (a) mechanical mixing and (b) ball milling homogenization [18, 19]

Works [18] and [19] describe SEM images depicting the decomposition of halloysite nanotubes in epoxy nanocomposites using two different homogenization methods (Fig. 15). The authors proved the superiority of ball milling over mechanical mixing homogenization. When the latter method was used, SEM images showed nanotube agglomerates of considerable size.

116 6. Conclusion

We may assume that the presence of agglomerates during the halloysite nanotubes manufacturing of nanocomposites is not unusual. Many papers describe the presence of such nanotube clusters reaching even several dozen micrometres in size and still define the dispersing level as good.

On the basis of the conducted microscope observations it is not possible to state clearly how compatibilizer influences the correctness of nanotubes distribution in polyethylene matrix. In specimens with compatibilizer and in those without its addition, agglomerates of nanotubes appeared and they were of considerable sizes in relation to the sizes quoted by the producer. The degree of homogenization of the obtained polymer material is not high. Also the adhesion of nanotubes to polymer surface is poor but this statement requires an additional research to be confirmed.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 734205 – H2020-MSCA-RISE-2017.

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Ivan Gajdoš⁵, Emil Spišák⁵, František Greškovič⁵, Janusz Sikora⁶