PSI - Issue 32

Kseniia A. Mokhireva et al. / Procedia Structural Integrity 32 (2021) 137–143 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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separate particle of OCSiAl Tuball nanotubes is a graphene plane rolled into a cylinder with an average outer diameter of about 1.6 nm and a length of 5 μm (Stepina et al. (2019)) . However, it should be noted here that, prior to the incorporation of filler particles into the matrix, nanotubes turned to agglomerate and tangled into bundles; see, for instance, Neverovskaya et al. (2019); Stepina et al. (2019). So, the filler underwent additional purification before it was added to the matrix. In the initial dry powder, as mentioned by Neverovskaya et al. (2019), the nanotubes content was only 75% by weight, and the remaining 25% included inorganic impurities (9 wt%) and amorphous carbon (16 wt%). These impurities could significantly affect the composite properties, and therefore the nanotubes need to be modified for their future practical application. The nanotubes were purified during the self-propagating high-temperature synthesis. Under combined action of temperatures and an aggressive oxidizing environment, carbon impurities went into gas phase and then were removed from the reaction zone, and metal impurities – into water-soluble form and then were washed with water (Neverovskaya et al. (2019)). Figure 1 presents the result of nanotubes dispersion.

Fig. 1. Electron micrograph of the initial dry powder of SWCNT (a) and the ensemble of modified SWCNT (b). Insert in the upper right angle – SEM image of nanotubes provided by the manufacturer.

It is seen that, although the SWCNT aggregates were not dispersed into individual nanotubes (see Fig. 1), the proposed approach can be used to produce nanotubes with a higher degree of dispersion. It should also be noted that the ensemble of the obtained nanotubes is free of both inorganic and unstructured carbon impurities, as pointed by Neverovskaya et al. (2019). Multi-walled carbon nanotubes (MWCNT) have a fibrous structure as well, but the average diameter of a single tube is 10 nm, and the number of walls varies from 7 to 9 (Bocharov et al. (2018)). The original MWCNTs contained amorphous carbon impurities (15 wt %) and metal catalyst impurities (1.5 wt %). These particles were purified by the SHS method as well. The filler particles introduced into the matrix were both original and modified. Rubber compounds with the filler content of 7 phr and 50 phr were produced using mixing mills. In highly filled vulcanizates, the filler composition involved 43 phr of carbon black and 7 phr of the nanocarbons. The use of a rotorless rheometer MDR-2000 made it possible to determine the vulcanization characteristics for all rubber compounds at 160 °C. The vulcanization process was carried out at 145 °C for 35 and 50 min in a hydraulic press at 18 - 20 MPa. 3. Results 3.1. Initial anisotropy The evaluation of the initial strain-stress properties and possible initial anisotropy of vulcanizates was carried out on a Zwick biaxial testing machine. In the experiments, the dumbbell specimens of the ISO 37-4 type were deformed. The dumbbells were cut in the calendaring direction and in the direction transversal to the calendering. They were subjected to tension until rupture; the distance between the grips increased at a rate of 100%/min. The