(259h) Next-Generation of Thermoplastic Nanocomposites Via Electromagnetic Processing

Carbon nanotubes (CNTs) undergo intense heating when exposed to microwaves, i.e., ~ 2000⁰C, due to complex conductive and dielectric lossy mechanisms¹. Polymers, in contrast, are virtually transparent to electromagnetic radiation in the microwave range and cannot be significantly heated by electromagnetic irradiation^{2, 3}. Is it then possible to harness such an intense heat from carbon nanoparticles into the processing of thermoplastic composites? Yes! In our previous work we have developed an environmentally friendlier and cost-effective method for fast processing of nanocomposites with tailorable microstructure and high dispersion levels, while displaying atypical combinations of high transport & mechanical properties⁴. Nanocomposites prepared using microwave radiation of 2.45 GHz for a few seconds with polypropylene polymer (Matrix Polymers, Revolve® PP 46) and commercial carbon nanotubes (Nanocyl NC7000) at a loading of 1.5 wt% displayed a volume electrical conductivity of 10^-3 S/m, which is about 2 order of magnitude higher than their conventionally processed counterparts at the same concentration. Such nanocomposites at 1.5 wt% also displayed a tensile strength of about 36 MPa, which is 30% significantly higher than the conventionally processed nanocomposites⁴. The formation and retention of a fine electromechanical network of CNTs around the polymer micro-pellets (by dry-mixing CNTs and micro-pellets) not only makes the mixtures electromagnetically susceptible, but is also believed to be responsible for the outstanding combination of properties and dispersion. The dry-mixing step enables control of the homogeneity, exfoliation and pre-dispersion of the CNTs in the green bodies, and ultimately of the final microstructure of the consolidated nanocomposites. Nevertheless, the green network is partially distorted during the irradiation step, whose physical phenomena is quite complex and convoluted. During the irradiation step we can find: (1) the generation of localized heat at the nanoparticles network layer, (2) the transfer of such a heat into the bulk polymer of the micro-pellets, which causes (3) melting, (4) coalescence of micro-pellets and engulfing of CNTs by the matrix, and (5) subsequently viscoelastic melt-flow. Such phenomena partially affect the network and microstructure to an extent that is currently unknown. Therefore, in this study we intend to present this novel processing method and the latest progress on the interfacial and transport phenomena that take place during the irradiation step and its effect on the network.

References

1. H. Al-Saleh and U. Sundararaj, “Electromagnetic interference shielding mechanisms of CNT/polymer composites,” Carbon, vol. 47, pp. 1738–1746, 2009.
2. Wu, Y. Pan, E. Liu, and L. Li, “Carbon nanotube/polypropylene composite particles for microwave welding,” Journal of Applied Polymer Science, vol. 126, no. S2, pp. E283–E289, May 2012, doi: https://doi.org/10.1002/app.36832.
3. R. Mishra and A. K. Sharma, “Microwave–material interaction phenomena: Heating mechanisms, challenges and opportunities in material processing,” Composites Part A: Applied Science and Manufacturing, vol. 81, pp. 78–97, 2016, doi: https://doi.org/10.1016/j.compositesa.2015.10.035.
4. Villacorta, Z. Zhu, R. Truss, A. Larsen, and G. Solomon, “Method for Fabricating Carbon Nanoparticle Polymer Matrix Composites Using Electromagnetic Irradiation,” US20200148853A1, May 14, 2020